Chopping is a defensive shot that depends on variation of underspin to force an error from the opponent. Some players choose it as their main tactic. Their style depends mostly on chopping every ping pong ball and waiting for the attacker to misread the spin. Occasionally even an attack player will find himself in a precarious situation from which he can only recover by chopping.
You only chop when you're far in the back court. Raise the table tennis racket. Let the ball come to you and strike down and forward as though you were trying to slice off a piece of the ball. Keep the ball low. Recover and resume a position in the center of the back court to be able to cover shots to either side.