A power essential: forearm rotation
Your hand can't turn over without the aid of your forearm, so you shouldn't concentrate on your hands to rotate the clubhead; your focus should be on your forearms. To prove this, extend your left arm in front of you with your palm facing the ground.
Hold your left wrist very tightly with your right hand so that you immobilize your left forearm. Now, with your right hand resisting, try to rotate your left palm to the sky. With your forearm immobilized, you can't.
The forearm rotates the hand, and in the golf swing, this action squares the clubface to the target. If you use your hands to square up the clubface instead of allowing your forearms to do it, chances are you'll collapse your left wrist. Here's a drill to help you get the feel.
Graduated release drill
1. Tee up a ball using your highest-lofted fairway wood (a 5-wood is perfect), then take your normal address position except that your hands are split on the grip with about 4 inches between them.
2. Cock your wrists and turn your shoulders until your left arm is parallel with the ground, over your toe line.
3. Now drop your arms down to the position they held at address, but keep your right wrist bent. Stop just before contact and note that the clubface is pointing to the right -- it's open.
4. Go back to address and do it again, but add a slight amount of forearm rotation. Note that just before contact, the clubface now looks at the target, i.e., it's square.
5. Hit some balls with this half-swing and split grip, then hit some with your normal grip, focusing on the correct release. If you start to lose the feel, go back through the steps of the drill.