Forgive me for wasting your time as a player! Releasing spores causes mushrooms to appear on the ground, then touching one of them will give you a jump and speed boost lasting 2 seconds so you can get to hard-to-reach areas. Then, that weird E thing does actually do something. You press E to turn into electricity and then hold A/D to move back and forward along a spline through the soil to the next platforming area. Doing so costs spores, the little blue things you collect in the overground areas.
However, I did absolutely nothing to signpost this, so I'm sorry. I had to cut my jam short by two days and didn't get a chance! Hoping to add some stuff in this week.