the same process they use for green screen if you watch the extras on the film the boards are painted green so they can be edited out its kinda cunning
ya the kids who make youtube comments are pretty retarted sometimes especialy in this video "they are riding bright green skateboards infront of a green screen you fucking idiot how else would they do it"
soooo true. obviously they went into the future and bought invisible skate boards and used them. the only shitty part is the fact that u lose the boards really easy.