Anti-spin rubbers have practically no grip (in some cases absolutely none) and this has very confusing effects on the ball. The ball's spin will come back reversed. So if the opponent gives you topspin, and you block with an anti-spin rubber, it will return to him/her as underspin. Many players are confused by this effect and will misplay the ball, either popping it up high for an easy smash, or putting it into the net.