Keep the trail knee flexed instead of straightening it
The coach's main point: keep the right (trail) knee flexed and turn, rather than straightening it. He says many people straighten the trail knee thinking it gets them more movement, but the right knee flex does not mean restricting the hips. Keeping the knee flexed lets the swing slowly build ground pressure from the foot into the heel, staying braced on the inside of the right foot, with no explosive straightening of the trail leg.
Channel: PorzakGolf
Watch official source at 3:32Why this context matters
Framed as a mistake to avoid: straightening or hyperextending the trail knee in the backswing.