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The Antigravity Plate was invented by the UNSC. It was designed to create Antigravity, but was used by the United Rebel Front during the Raid on Camp New Hope to create 10Gs of pressure, thereby incapacitating a group of Spartans. For this to happen, the plate must have been mounted above rather than below the Spartans.
As of 2531, it was old technology, and they never were widespread, but some were still around. What exactly they were intended to be used for was never specifically stated, but it is possible that they were to be used to lift heavy ordinance by causing it to float, lifting it up, and making it levitate for easy transport. UNSC starships clearly have a form of Antigravity, demonstrated by their ability to hover in mid air without rocket propulsion being directed below. It's possible that only very large power plants can generate the power needed to produce antigravity, and the antigravity plate may be an early attempt to miniaturize the system found on starships.
It's unlikely that this technology is based on magnetism, as such a device would be extremely dangerous around metal objects commonly found on UNSC ships. It's more likely that both gravity and antigravity devices somehow convert energy into gravitons and antigraviton particles respectively, which are then focused into a highly directional beam. This would mean the human and Covenant technology were not that far apart in this regard, with humans lacking the technology needed to miniaturize gravity/antigravity generators. This would also explain why the UNSC was able to so rapidly reverse engineer and implement artificial gravity systems on board their starships; they may have been on the verge of doing so already. With the influx of technology in the post-war era it would make sense for all UNSC ships to have some form of gravity on all ships.
In 2531, the United Rebel Front acquired one of these and used it in an elaborate attempt to capture a team of Spartan-IIs during the Raid on Camp New Hope. With a focused beam from the plate, the MJOLNIR Armor sensor systems were tricked into thinking they were in a 10-G environment. The armor increased internal pressure to compensate and rendered them unconscious, allowing the rebels to capture them with Neural Inhibitor Collars. Although the tactic was remarkably effective, it failed because Kurt-051 had not been caught in the ambush, and was able to free his fellow Spartan-II's.