Racket Head Speed
By Carl Danner

For maximum spin, you need maximum racket head speed. The way to achieve this is to brush the ball with a loose wrist and forearm. By contrast, tightening your muscles will stiffen the movement, slow down the racket head and reduce the spin.

Also, for full strokes (such as a loop), you want to make the biggest swing arc possible consistent with your ability to control the stroke. This means, on the forehand side, contacting the ball further away from your body and swinging from the shoulder. If you swing instead from the wrist or elbow, you reduce the size of the arc and your racket will slow down considerably.