Orient the lead thumb to leverage the face on plane
The instructor uses a grip with a thumb marking and flat spot to establish lead-hand position. He says that when the thumb is correctly oriented to the shaft, the club can leverage against it in the backswing and put the face on plane with the back of the lead hand in the demonstrated flat orientation rather than rolled.
Channel: Todd Graves
Watch official source at 7:26Why this context matters
The checkpoint is demonstrated with a molded training grip.