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, bar E.T.A.I and Miyu. The girls are put in such a position they lie on their chests, with their torsos inclined down and their asses sticking out, their legs and hands hanging freely. The slab has small promontories which are used to rest the girl's heads, and maybe collect the drool coming from their mouth, so as to avoid making a mess. It is seen for the first and only time at the end of Episode 3 of Aika Zero. However, there was a similar scene earlier in this episode in which there was another slab used similarly to drape the captive girls; this is beyond the door to which Rin's butt was the key.
This is also a notable source of fan-service because the metal slab scene exposes the panties and butts of most of the White Knights all at once. 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 advances across the defeated (and humiliated) girls' butts to show off their - as Eri puts it - "beautiful skin". The defeated girls also seem to have bigger butts in this scene than any other point in the anime, however, this could just be down to the particular position and camera perspective. The primary purpose of the metal slab seems to be to humiliate and punish, as well as keep the girls 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 be able 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).