Training tool for tennis: Pro Trainer WinSock
"For beginners," says WinSock inventor/owner Bill Gibson, "it is very useful in learning to execute a full finish on ground strokes, to let the racket turn them in recovery to the center, and to develop upper body strength. [The device] delivers a powerful and reliable swing movement and dramatically improves swing speed and swing mechanics."
Gibson recommends short, moderate speed sessions of 10 to 20 strokes (each) for best results at all levels.
So, how does it work? The added 8 ozs. of weight, equally distributed at both sides of the racquet throat, necessitates that you hold your racquet more firmly and in the correct position while hitting. Use it to groove groundies or volleys and to develop strength, muscle memory and faster, controlled racquet speed that can help generate more spin and power.
I found the WinSock easy to put on and take off, simple to work into a training regime in the recommended small doses. You do not need to hit hard to reap the benefits. Go easy to begin, especially if you do not lift/work out regularly. One extra note, the company recommends that you do not serve using WinSock. The service motion, however, is ok.
Find it at PTWinSock, $30 USD. Not recommended for children using kid sized racquets as young muscles are not developed enough to handle the extra weight.