Excellent piloting, very skillful. From what I can tell it simply untethers an FPS limit with the engine and the game starts running at twice the speed. I've read on different forums the v-sync in Game Maker isn't "true" v-sync, and is just the engine slowing the game down by a few milliseconds to stitch together inbetween frames.
It's a shame since from what I can tell looking at different videos, the autocannon shoots much slower than it should. It should fire at just below the firing rate of the minigun. Not much I can do on my end to fix it sadly. I might end up porting the game to GMS2 if there's more compatibility options from there, but it'll be awhile until it's finished.