Strong trail-hand grip and a shut clubface
The speaker says a strong right-hand grip, especially with the trail hand, shuts the clubface. A swing where the person lets it go will hook, while hanging on because the person knows the face is shut is the bad swing.
Channel: PorzakGolf
Watch official source at 0:48Why this context matters
From a driver lesson diagnosing a student whose strong trail-hand grip hooks the ball.