Flattening the wrist at the top closes a strong-grip face
With a strong grip the wrist is cupped at address; trying to get the wrist flat at the top of the backswing naturally closes the club, because you started from that cupped position. The speaker says this is a huge change in club face angle that de-lofts the club and points it low left, making it hard to hit the ball high and producing low shots and hooks.
Channel: SagutoGolf
Watch official source at 1:10