What I expected to happen was...:
Push me away/ up
What actually happened was...:
Let me go swim, on ground
Steps to Reproduce:
1) make a cave with 2 blocks high and make the ceiling normal pistons,
2) activer the normal piston when your in it, with redstone.
3) go in F5 and look yourself swimming in no water.
I believe it is an adaptive gliding mode instead of swimming. You can achieve that by also spam jumping into a 1-high gap.