Use the up and down keys to make the skate boarder stand up or squat. Use the left and right keys to decrease or increase friction. The "pump" technique used in order to keep the skate boarder going up and down is to stand-up (up arrow key) when the skate boarder has high angular velocity (e.g. when approaching the flat surface) and to squat when the skate boarder has low angular velocity (e.g. when it is getting close to the top of the semi-circles, just before it starts to move in the opposite direction)
The mini-ramp is a nice example of angular momentum conservation. The objective of the skate boarder is to continuously go up and down the mini ramp even though it is subject to friction. This can be achieved by having the skate boarder change its center of gravity in relation to the center of the mini-ramp semi-circles (left and right) by moving the skate boarder up and down. Standing up, decreases the distance to the center, thus increasing angular velocity and vice versa. Thus the idea of the "pump" is to squat when angular velocity is low and stand-up when angular velocity is high (remember angular momentum conservation implies that w(t=2) = w(t=1) * r1^2/r2^2) ).
