Antigravity plate

The Antigravity plate is a form of anti-gravity technology developed by the UNSC. Originally designed to counteract gravity, it 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.

Background
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. UNSC starships clearly have a form of antigravity, demonstrated by their ability to hover in mid air without rocket propulsion being directed below.

It is 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.

Operational History
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.