An Impossible is a trick where the board performs full backflip. There should be no sideways rotation like a heel or kickflip but rather a "nose over tail" type 360 degree rotation. It is easiest to perform this trick in the skatepark Inbound, but can be performed anywhere.
The Impossible is easiest performed out of a shallow ramp. Swipe down on the board, as if to do an Ollie, then carry quickly carry the swipe back upwards. The Impossible is fairly simple so with practice the it becomes quite easy. Don't lift your finger up.