Simon, I think this is another area where you find a lot of variation. Some golfers have a pretty deep knee flex and maintain it throughout the swing. I found that I was doing just what you're doing: rising up--not on the backswing but on the downswing. I reasoned that if I'd start with straighter legs, that'd solve the problem, and it did. I don't keep my legs dead straight, of course. I keep enough flex in my knees to "unlock" them, but not much more. And I cock the right knee toward the ball a bit, to pre-load the hip turn and prevent reverse pivot--and it just feels right to do this.

Experiment, and don't feel that an "athletic" stance necessarily means a tennis crouch.

__________________
Todd
Philadelphia, PA
USA

The reason the pro tells you to keep your head down is so you can't see him laughing. ~Phyllis Diller