Not "rolling over" your hands does cause a slice as the clubface never gets closed through impact.
Keep your left arm straight. Everyone always says not to "lock it out" and a "little bend is okay", but from the slo-mo vids of the pros, they most all lock it out.
The less moving parts, the less that can go wrong.
Your hands should roll over through impact naturally due to momentum. If they don't, you are applying forces to STOP the rolling over. Either "holding on" or too tight of grip pressure can cause this.
To get an exagerated feel of this, hold your club with your normal grip. Leave the clubhead on the ground and rotate the club so your right hand (for a rightie) faces away from your body. The lines on the clubface should now be 90* "open" and parallel to your target line.
Now, still leaving the club in contact with the ground, ROTATE the club so that your LEFT palm faces away from your body. Do you see how your right hand has "rolled over" your left? In doing so, you see the clubface go from 90* open, to square, to 90* closed.
This is an exageration of what your hands, and clubface, SHOULD be doing through impact. Though it SHOULD happen with no real concious effort. And it will if you relax your wrists and forearms and let the club "swing".
Now you'll end up hooking the piss out of the ball and come back asking how to cure your hook.
