Stability and Power
Stability is crucial to controlling and delivering power to the ball through the racquet head. Any small wobble in the racquet during the stroke will, at high speeds, translate into a very large error. Instabilities are the most common source of unforced errors in a match. The rollover animation below is one example of stable movement.
Move Cursor over Image to see transition.

Move the cursor back and forth over the image and observe how the racquet is advanced. While it appears that the player is pulling the racquet forward by the butt, this is actually an illusion. The movement is more closely related to a boxing punch.

The sequence below is courtesy of ProCompare Tennis. It illustrates good stability as the player maintains balance through out the stroke sequence. Original full size photo and video can be seen at procomparetennis.net.