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.
Environment
operating system: Windows 7
I believe it is an adaptive gliding mode instead of swimming. You can achieve that by also spam jumping into a 1-high gap.