[chuffwaffle]

I can tell you how it works. You maintain your initial jumping velocity almost all the way up to the top of your jump. The peak of the jump occurs suddenly, and it's when you release the jump button or when the maximum jump-height is reached, whichever comes first. The maximum jump-height increases by a blockheight or two if you were sprinting when you jumped.

The only part that I'm not too sure about is what exactly happens to your velocity when you start to slow down at the peak of your jump. You might linearly decelerate like you'd expect from gravity, but it might be less newtonian than that; it's hard to tell just from the feel of the game.