I used to own a half-pipe. Well, a mini-pipe rather. It was a about 1.5 meters tall.
Skate baording on it is interesting because at the top of the pipe all you're energy is due to gravity.
That means Etop=mgh
As one rides down the half-pipe, potential energy is converted to kinetic.
At the bottom Ebot=(1/2)mv^2
In many sports that include a standing on board, a common method to gain speed is to PUMP. Pumping, in its simplest form, is pushing down on the board when you're going up or down a ramp. Or any curve for that matter.
Its possible to PUMP on any curve who's concavity faces upwards.
In the case of a half-pipe: one can pump on there way down the pipe, thus converting energy in their legs to kinetic energy using an impulse (push). And it works too. Its actually quite crucial while skating on a mini pipe.