Topspin is a ball that rotates forward from top to bottom in the same direction that the ball is traveling.
Friction is an essential element in creating topspin. Try getting topspin on a ball with a 2x4 instead of the grids of a racket string bed and you'll see that friction matters. Beyond that, the rate of rotation, speed, and height are determined by racket head speed at contact and the angle of attack on the ball.
The Original Standard Method created by Dennis Van Der Meer described a topspin groundstroke simply as generating a low-to-high motion with a vertical racket face.
