[Mike O]

Mike, a square-to-the-arc clubhead means that the clubface is perpendicular to the swing arc. The forearm rotation clockwise during backswing opens the clubface in relation to the arc; the same rotation but anti-clockwise brings it to the square position during downswing and closes it in the follow through.
Now, depending on the release type, some golfers brings the clubface square earlier and some later just before impact. The same may happen after impact - some golfers maintain the clubface square after impact (those who swing left with a late swivel) and some close it rapidly just after impact together with a crossover of their forearms (early swivel).
Hogan delivered his clubface square already before entering the impact zone and maintained it square long after impact what is one of very suspected "culprits" of his overhuman accuracy and repeatability.

Cheers

Thanks for your post- you described the concept of square well enough for me to grasp it. Obviously, I'm assuming that when we say "clubface square to the arc" we mean the leading edge (as clubface loft would create a whole different set of problems). Also, if as you describe- that before, during and after Hogan's impact his leading edge was square to the arc- then we are merely saying that he had (in TGM terms) angled hinging- correct? (Since during impact I imagine angled hinging as having the leading edge staying square to the arc.)
Horizontal hinging and vertical hinging would only have one point where the leading edge is square to the arc through the impact period. So given your post you're implying that angled hinging has a superior accuracy and repeatability- which doesn't make sense to me. Food for thought!