Overlap to limit wrist hinge and square the face
The speaker describes bringing the wrist joints close together - the biomechanics he attributes to Harry Vardon and Mo Norman - as creating a limit on how far the wrists can hinge in the backswing. With a 10-finger grip more wrist angle is possible, whereas here the trail hand blocks the lead hand so it can only go so far. He says this hinge limit carries down so the clubface squares up more easily.
Channel: Todd Graves
Watch official source at 3:38Why this context matters
The core argument of the video for why the overlapping grip helps ball striking.