Here is an explanation on how to perform the glitch, from the discord:

" The glitch was discovered by samman789 Basically you tap the jump button (the up arrow key) extremely lightly and extremely fast. The character should jump more lightly. When you notice that, you press jump again inmediately while on air. That should do the double jump. It's similar to how you play a staccato note on piano: You just press and release as fast as possible without making too much force. It's hard to get it consistent but once you kinda figure it out you can skip pretty much the whole level using it. There was a video showing how to do it but it got deleted sadly. "