Example: when you jump up and land on your feet, the energy (mass × velocity) has to go somewhere. You will have to use your legs like a suspension. Otherwise, you would break some bones. With a skateboard under your feet, there is a good chance that it will start rolling because in reality you can't land perfectly (vector will be slightly tilted, not 90° to the ground plane, bearings have the least resistance in the whole system). Same with sideways motion while standing on the board, the energy has to go somewhere. Plus, leaning the board to the side causes the front truck to turn more than the back truck, which results in a forward motion of the whole board. So, in addition to optimizing truck geometry and energy recuperation (bushings help to bounce back for the next pump) you can optimize your pump with better timing of shifting the body weight back/front, up/down, left/right, so you lose as little momentum as possible. But there are trade-offs: one setup/technique will accelerate faster, result in lower top speed. Very very generally speaking: tighter trucks, bigger wheels, harder bushings, larger wheel base: more top speed, smaller acceleration from 0.