THIS GLITCH IS VERY HARD AND POSSIBLY FRAME PERFECT, I RECOMMEND ONLY DOING IT IN IL'S OR REALLY OPTIMIZED FULL GAME RUNS

So anyway, here's how to do this glitch, it involves intentionally lagging the game by holding the game window like in boss skip, and the glitchy jump caused by holding jump for a specific amount of frames. So you have to intentionally get the glitchy jump, and then lag the game the exact frame before you leave the ground. I recommend practicing this on the high up platform in level 2, I have a video here to demonstrate what the trick looks like. I don't know how to explain this any better, but if you just keep trying, you'll get it eventually.

can you explain it more in terms of like, inputs? when do you grab the window, how long do you hold jump for, when can you let go of the window and/or jump?

you jump and hold the window at either the same time, or you jump, and then hold the window a frame later. And also the glitch most often works when I don’t hold jump for that long either.