Hip turn | Golf Monthly
but my speaking to legs straightening up prematurely was about sometime in the downswing prior to impact {though it's true that sometimes during the backswing a locking out or over straightening of the trail leg can be an issue in a swing
would be careful at set-up that the legs are not over bent - ideally there just want to be a tad of flex in both knees so still a felling of being 'tall' but in an athletic posture
as to the other post comments about what Rosey was doing at transition putting forwards the ideas that JR had no small lateral bump before the lead hip clears - take a look at this real slo-mo vid of a current (May 2017) Rosey face-on driver swing & look closely at the hip motion at transition against the 90º vertical red line I have drawn in just on his lead hip can see the small lateral hip bump as he puts vertical pressure back into the lead leg/foot - before the lead side clears against the lead leg
also put a marker on the outside of his trail hip to highlight how there is no lateral slide to the trailside during the takeback & backswing
[video=youtube;u6_DBc1NOpM]
in biomechanics what's happening in the pelvic girdle & hip sockets (hips) during the backswing/transition/downswing
the trail 'hip' as the backswing progresses goes into internal rotation with the lead hip staying externally rotated
- at transition the trail hip moves to being externally rotated as the lead hip also is still in external rotation - so dual external rotation - this is the reason folks see the 'squat' type motion that takes place during most high swing motions
- as the downswing motion continues so somewheres between 3/4's & 1/2 ways down the trail hip then the lead hip both go into dual internal rotation before the lead side clears against a firm ('posted') lead leg
