The amount of momentum before and after a jump is equal. Prior to jumping, neither the skateboarder nor the skateboard had any momentum. Following a jump, the skateboarder gains forward momentum, thus the skateboard must also gain reverse momentum
In the hippie jump, a skateboarder travels at a specific speed Vh along a flat, horizontal surface. Then, without touching the board horizontally, he jumps straight up. He may now soar through the air at a speed equal to that of the board (also moving at Vh). As a result, he can land on top of the board because it is still right underneath him. this is how the momentum involved.
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