Perhaps the most iconic scene of the Aika Zero OVA, the metal slab is used to drape the unconscious bodies of all of the White Knights after they have been put down, bar E.T.A.I and Miyu who are retrieved by Neena Hagen. It is a metal devive with the shape of a trapezoid, the upper side having a noticeably tilted surface, with one edge much higher than the other, and whose lower side ends in a set of wheels for easy movement. The apparatus is completed with a number of small promontories which emerge from its shorter side, extending in the direction of the upper surface.
The metal slab is probably Eri's answer to all the amount of girl carrying she often sees her people forced to perform. Already in Aika R-16, in both episodes 1 and 2 there's a bunch of moments in which knocked out enemies and even friends must be moved from one place to the other. In Aika Zero this trend continues, and it's only logical Eri or someone in her team eventually realized they were really in need of finding a better way to at least move many girls at once for saving time. This is probably the origin of the great invention which the metal slab is, making it's debut in Episode 3 of the last series, first for retrieving the kidnapped girls and then for placing the defeated White Knights which they have under their custody.
The girls are put in such a position they lie slanted on their chests, with their torsos inclined down, resting on the tilted upper surface. Their heads face the shorter side, where they end up resting on the small promontories the slab has, while their arms hang freely to the sides. It's a distinct possibility these projections are also intended to collect the abundant drool coming from the unconscious girls' mouths, so as to avoid making a mess while moving them. On the opposite side, their hips rest on the higher edge, legs also dangling freely, in a posture which causes the girl's butts to prominently stick out, strongly bent at the waist as they are.
The metal slab is also a memorable source of fan-service, as the scene in which it appears allows to expose the panties and butts of most of the White Knights all at once in a rather humilliating manner. This is due to the fact that the girls all wear very short, basically hip-length, dresses and they are bent over the slab in such a way their skimpy uniforms end up flipped up to around breast-level, leaving their pantied posteriors fully exposed and clear to see. In addition, the camera does not refrain from dutifully panning across the defeated and humiliated girls' butts to show off their - as Eri puts it - "beautiful skin". The White Knights also seem to have bigger butts in this scene than at any other point in the anime, though this could just be down to the particular position and camera perspective intended for high viewer enjoyment.
Besides being a means of transport, the primary purpose of the metal slab seems to be to humiliate and punish, as well as keep the girls accounted for in one place until a more suitable punishment is formulated (at one point, Karen mentions that "bad girls will be punished"). Eri jokes (or perhaps not) that this is the captain's "chance to do whatever you want with them" but the captain quickly shoots the idea down.
It is, however, also a fairly suitable way of keeping the captives, who are still fairly dangerous if left unchecked, under control. This is due to the fact that any girl who starts to wake up, will intially be fairly dazzled and confused and will require some time to slowly regain her senses as she realizes where she is. Her first reaction will thus be to try to lift her head, check her surroundings and then try to get down from the slab. In the meantime, any person conveniently located behind the slab will have plenty of time to detect the movement while remaining unnoticed. Thus, as the unsuspecting girl is just slowly trying to regain her composture, the guard can hastily put her back down with a quickly delivered dose of tranquilising liquid to her wide open and defenseless butt before she even knows what's happening. The posteriors being quite large and fully exposed, it's an unmissable shot. This is precisely the sad fate that Kana and Rin experience, and probably other girls too, only that we never get to see it.
Kana and Rin both wake up for a few seconds while being in the slab, only to be hastily put down again by Karen with two well aimed shots to each of their butts. Upon impact, the helpless girls arch their backs and raise their butts for a moment, while letting out two barely audible screams. They remain shocked in this position for some seconds, their mouths wide open and drooling, before collapsing once more, face first and butts following soon after. This rather merciless action with two defenseless and already defeated foes is seen by Karen as the beginning of the righteous punishment for the girl's previous deeds. It can be expected any other girls who happens to wake up in the future will receive the same rough treatment, for as long as Karen deems necessary for them to learn their lesson.
From left to right on the image of all of the White knights draped: Reiko, Misumi, Manami, Rin, Kana, Misaki, Nanami and Yui (Karen is poking the butt of Kana with a gun).
White Knight Expo Scene
As Eri and Karen are casually talking about the happy end of their adventure, the Captain remembers them that they still have one fairly voluptous detail left to attend to, as he points them at a spot behind their location. The camera then moves away from the group and reveals the retrieved bodies of the captured White Knights, which have been laid out on the metal slab. All the girls seem to be deeply unconscious after having been knocked out, the camera panning over their seemingly peaceful faces, where we can appreciate their mouths are open and profusely drooling.
Taking advantage of the unique situation, the camera indulges in plenty of new views of the defeated girls. First it pans across them from the front, showing their faces in the foreground and behind their torsos as they go up the slab, ending in the girls' prominent hips and butts cut against the sky. All the girls have their school uniforms flipped up to around breast-level, exposing their backs where the dimples of Venus are noticeable. Then, it goes for a full view from behind, moving slowly across their fully exposed rears from a downhill perspective, starting by Reiko, who gets the absolute spotlight and is the only one whose rest of the body can at least be partially seen.
Once it finishes with the captives inspection, the camera then misteriously moves to get a view of Kana and Rin's faces as they rest on the slab, two thick lines of drool surging from their gaping mouths. Is is then than we realize some of the captives might not be as done with as we could have initially thought, just as Rin lets out a small whimper and her eyes start flickering, to out great delight. The White Knight then slowly but surely opens her eyes and lifts her head a little, with a drowsy and confused look on her face. It is not long before the same process repeats with Kana behind her, who also follows her friend in lazily waking up and lifting her head as she tries to understand what is going on. Thus both girls are soon discretely conscious once again, still dazzled and confused but a high level threat if left unattended and allowed to recover.
The camera doesn't stay where it is for long though, and soon moves to locate itself right behing the gradually recovering girls, conveniently aligning itself with their exposed rears and getting a frontal view of Rin's butt as it rests on the slab. It is then than, unnoticed to the poor girls who are still silently struggling to regain their composture, we are able to see the shape of a tranquilizer gun emerge from the bottom, hastily locking up on Rin's wide open ass before mercilessly shooting her without any warning. Defenseless, the shot is a direct hit on the defeated girl, who can only jerk in response to the sudden impact as she lets out a sad soft moan. Her ass first bounces down as it takes the hit, only for later soaring up as Rin's back involuntarily arches, all muscles going tense in response to the unexpected attack. We can then appreciate how this movement lets exposed the front of her panties, where a tiny yellow ribbon can be seen.
Done with Rin, the gun then quickly moves to deal with Kana, with the camera following it in it's ominous trajectory. The remaining girl, probably still too drowsy to understand what's happening, remains oblivious to the sad fate of her friend and so emits no sound, nor perfoms any attempt at a defensive manouver. Like a sitting duck, she just stays silent and inmobile, unaware it's now her turn. The gun again shows no mercy, shooting Kana in the same fashion as it did with Rin before, from point blank range. Once more, the helpless girl can do nothing to avoid her fate and just jerks uncontrollably as she takes in the hit. Her ass goes down as the tranquilizer dart gets embedded in her rear, later bouncing up as Kana emits a barely audible pitiful whimper. Like Rin, she ends totally tense, muscles strained and back involuntarily arched, keeping her ass high up and sticking out, though slighly slanted. We can then appreciate the ribbon which decorates her panties is pink.
Both girls having been taken care of, the camera returns to a frontal view to show us the aftermath. Kana and Rin are both in clear shock, their heads lifted in the air, whole bodies trembling as they pour out a series of soft heartbreaking whimpers. Their eyes are wide open, pupils vacantly fixed on an unknown distant point, and the lines of drooll surging from their now wide open mouths have gotten thicker. The shacking girls also exhibit a slight blush on their cheeks. They are, overall, a most pitiful sight.
The two White Knights maintain this strained posture for some seconds, while we can imagine the tranquilizing serum advances across their tense and witching bodies. Eventually, however, both go ominously silent as their eyes and mouths beging closing slightly. Their heads also go down a bit, as the girls start going back into the realms of unconsiousness and their strained muscles begin relaxing. Finally, the tranquilizing darts kick in for sure and the students' eyes close once more, their heads unceremoniously crumbling down inmediately after, ending up once again resting on the metal slab where they started less than a minute ago.
To wrap up the episode, the camera moves behind Kana and Rin once more to show us a last view of their butts. in this way we can appreciate that, after the collapse of their upper bodies, their posteriors still remain raised, buttchecks clutched tightly together. This does not last for long, though, and we soon see both asses go limp and unceremoniously crumble down, bouncing down a bit as the girl's hips hit the slab before going inmobile into their resting position.
The camera stops for a bit in this location, most probably in silent contemplation of the pitiful sight, before zooming out and revealing the merciless culprit behind the girl's cruel put down: none other than Karen, who argues "bad girls deserve punishment" as she ominosly looks at the fallen girls, or rather, at their wide open rears.
Looking behind the facts that the girls are already defeated and their young age may justify some nicer treatment aftet all they've already been through, Eri seems to think this is an appropiate procedure and just laughs off the whole ordeal, agreeing some kind of never specified "punishment" is in fact needed, leaving the actual meaning of her words to be left to the free interpretation of the listener. As she speaks, the camera takes a final panoramic view from behind at the slab with all the knocked down girls draped on it, their asses and legs in the spotlight. Though none move or emit sound, Karen watches closely over them, threateningly poking Kana's left buttcheck with the tip of her gun. It is only safe to assume the poor girl's punishment ain't even close to being over yet.