Here is how to Underflip.
A flip trick performed by flicking the skateboard from the underside of the skateboard. This can be achieved whilst on the ground by hooking a foot underneath the board (the original Kickflip, the classic flip, was essentially a type of underflip) or after a normal flip trick performed in the regular fashion in order to get into a position to underflip.
The most common of this type of underflip is performed as part of a kickflip-underflip. Riders kick slightly downwards on the kickflip instead of forwards and sideways, so that when the board comes round to finishing it's flip the front foot is underneath the board, in a position to quickly kick back up so it flips another full time in the other direction.
It can be hard to spot someone doing one of these, since the whole motion is really fast, but if you see someone doing a weird disjointed flip trick where the leg moves two ways, chances are it's an underflip of some kind.