Noticed this accidentally while trying to run follies.

If you dash or move forward ~300 - 320ms (yes I actually tested this with a script) after a jump to a ledge, you will "clip" under the ledge. This is fairly difficult to do perfectly each time, as it's just a tiny bit before a full jump.

After clipping, I tested spam-dashing a bunch of times, and the results were usually cuphead scrolling across the screen at faster than usual speeds, and then getting thrown back up. In rare cases cuphead was actually sent backwards before being tossed upwards.

I think this could be incorporated into at the very least follies to shave off some milliseconds in at least two places (since noone runs the other RnG levels in Any%), but probably not much more than that since you will take damage from the fall unless you combine it with the smoke grenade glitch (in which case someone better than me should test chaining ledges to scroll past the level at super speeds, but that sounds more like something a TAS would do than an actual human).

I think there is a lot of potential in this glitch, you get a huge speed boost right after clipping into the ground. I tried mashing dash and was able to maintain that speed for a screen length or so, but I wasn't able to reproduce it reliably. If you nail the trick first try you can skip acorn maker without slowing down at all.

here it is used to skip the acorn maker: https://gfycat.com/gifs/detail/ElasticIdealisticArcherfish right before the acorn maker on another grass thing: https://gfycat.com/gifs/detail/IncompatibleDependentAsiansmallclawedotter right at the beginning of forest follies: https://gfycat.com/gifs/detail/GrizzledVillainousCommongonolek

I was only able to clip into the dirt/grass platforms on forest follies, I think it may be a bug with that piece of terrain.