I've noticed that when my grip tightens (intentionally or not) in the down swing, more often than not, I will hook or draw the ball when I intended on a straight flight path.  So, I held my 7 iron straight out in front of me with the face perfectly perpendicular with the ground.  With my thumbs 1/4 inch back of the shaft centerline (in my case to the right) and my "Vs" pointed slightly between my chin and trail shoulder, I tightened my overlapping grip.  The face closed!  With my thumbs directly on center line, as hard as I squeezed, I could not close the face!  This may be a cure for the unintended hook or draw.  Has anyone tried this?