Everything is sped up: movement, rotation (ok maybe not rotation, you do need to be able to see), sound effects, weapons, reloads, it's like your computer's clock speed is getting faster and faster.
It keeps getting faster until it starts to approach un-playability. ie: eventually you are crashing into walls the instant you hit a movement button.
Then the level ends/resets.
Is that a thing already? Do any games do that?
I think it would be pretty funny.
Particularly in descent, where joystick movements are not typically that fast. I'd like to watch someone play descent with their joystick reduced to a twitchy blur

So yeah, i think it'd be an interesting mod for a lot of multiplayer games (remember it effects all players equally), particularly FPS games.
Single player games could get weird, since the AI will likely be at no such disadvantage in a sped up environment (unless we try to simulate lag for the AI, which could be an idea). Maybe the singleplayer mechanic would be have the speed be a dynamic thing, like a difficulty setting that changes as you play depending on how well you're doing. If you're doing well - you're playing too slow and it speeds up.
Anyway, that's a sidepoint. The multiplayer is what i was mainly thinking about.
And boy oh boy will it speed up your map learning ability. Infact that's prettymuch what the gameplay mechanic will devolve into, it forces you to react strategetically as fast as you possibly can. All gameplay eventually devolves into map knowledge, pavlovian responses, cause-effect, you'll soon be navigating levels via your muscle memory alone.
Each level will still start normal though. So you'll still be playing the same game for at least a few minutes, it'll just get gradually faster as the level goes on, forcing a gradual change in tactics until it's no longer anything like what you remember.