There seems to be a couple of approaches:
Upper Body Momentum High Jumpers: The most powerful looking method, utilizes jump momentum for height, with knees sucked up straight to varying degrees eg. Westgate, Reese
Flexy Boners: The more idiosyncratic approach perhaps, while momentum is used to gain height, the board is also elevated by a forwards and upwards boning motion, ideally peaking with the board and both legs nearly all parallel eg. Kenny Reed, That Hari guy, Levi Brown