In the old days this skateboard trick was called a Melancholy (probably because it rhymed well with ollie). It refers to a simple backside grab trick.
While you are getting a reasonable amount of air off a drop or on a ramp bend up your legs and reach back with your front hand and grab the heel edge of your board somewhere between the trucks. If you have plenty of hangtime tuck your trailing leg and extend your front leg like with a tailgrab. Pull back and watch your center of gravity for the landing. Snowboarders (of which I am not) do the trick the same way pushing off the tail of the board and grabbing their heel edge between the bindings.
For variations of this basic grab, tuck in the front knee and bone your trailing leg for a Mosquito. If you don't tuck at all and keep the chin-to-knee line flat you get a Method (arch further back for more difficulty). On a snowboard grab the heel edge with both hands for a Flying Squirrel (I wouldn't try it on a skateboard).
This move is easy to spin on a vert ramp, so you'll see plenty of 360 degree frontside melons.
If you know the true story of the name I'd like to add it here