When doing an original casper the skateboarder stands on the board so that the heel of his back foot is on the tail and the ball and toes are hanging from the side.
After balancing in this stance the skateboarder usually jumps up a bit, lifts his back foot from the board and uses his front foot to turn the board back to its normal position with a motion that resembles an impossible.
The board is turned upside down with the tip of the tail on the ground acting as a fulcrum, the front foot under the front of the board holding it up acting as the effort, and the back foot resting on top of the back truck as the load.
Exiting the trick can involve rotating, flipping or wrapping the board around the foot.
The reverse of this trick is the Anti-casper which is the same principle only applied a half-impossible into a casper on the nose of the board.