Hitting the tennis ball

Marian is hitting the tennis ball very well. Although it is a tennis ball on a string. It is quite remarkable how the ball flies off the racket. She is better than me at transferring her body weight into the ball. I have this awful habit of relying on the strength of my arm whereas with Marian, she has a proper back swing and follow through. All the more remarkable when she has only seen me hit the ball. How on earth could she have picked up this swing?