Topspin is a technique through which the player gives a rotation effect to the ball, which will make the ball create a nice curve towards the ground. The technique is actually performed by swinging the racquet across the boy, the motion has a similarity with a windshield wiper. A the end of the hit, the player has to accelerate the ball, using the wrist, that gives the final effect to the ball.
That is true. When tennis players apply top spin, they sweep their arm across an imaginary plane, rotating around their elbow, essentially how a windshield wiper moves, except without the glass, and the forearm as the wiper.
