To twiddle is to spin your racket in order to switch the backhand side of the racket to the forehand side of the racket. This is often used by defensive players who use a pimpled rubber on one side and a regular inverted rubber on the other side. In the middle of a fast rally, this player will twiddle and play the ball with the other rubber, hoping the opponent doesn't notice the change and is then likely to misplay the ball.
In an uncommon scenario, Ma Long (China), likes to twiddle when he is smashing
and the opponent is lobbing
. He uses his backhand rubber on the forehand side since it generates more power than his normal forehand rubber.