Your Knees - Less is More!
Your knees help to provide fluidity and power in your swing. But be careful! Many duffers flex the knees too much, resulting in a swing that is very hard to control.
Rotation, Weight Transfer, and Power
K. J. Choi has a wonderfully consistent swing. Watch the video below, and pay attention to how he systematically uses his knees to allow his hips to turn to the right on the upswing, and to the left toward the target on the follow through.