Pay attention not only how his clubface remains square to the arc but how his rear humerus work parallely with his hard pivoting body. No strange Jim Furyk is being regarded as the most consistent ballstriker of our era even by Trackman specialists.
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It is a great action. It is worth adding again that Mother Nature made the elbow joint the way it cannot move in all possible directions because it would be too weak to support a lot of physical activities correctly; therefore, hard structure needs additional motions of forearm to adjust the lack of RoM in the elbow joint to allow some physical actions to be achievable. One of this motion is pronation/supination of the forearm (turning the forearm axially without necessity of moving the elbow joint in space). Physics of golf swing must include pronation/supination (and bending the rear elbow as well, BTW) because otherwise one couldn't make a full swing.
Now, when rear forearm pronates (during downswing) it affect simultaneously the shaft and the clubface. It is much easier to control the clubface when shaft is aligned parallelly - or better said - is a parallel extension of pronating body part. Any angle between it (i.e. when shaft is not parallel) forces an additional action of wrist to adjust to impact.
Probably this is one of main reasons why the most consistent ballstrikers were elbow planers with their rear forearm supporting the shaft and rear humerus tight to the body. Other reason was that their pivot was great enough to let the above occur. It requires the rear elbow joint be on the rear hip at impact. The more the elbow goes in front of the body the worse for the whole motion. People often say "stuck" incorrectly.The rule of thumb is -- the stronger the pivot is the more open is main body at impact (hips more, shoulder girldle less of course due to sequentiality) and the more is the lead arm pinned accross the chest while the rear elbow close to the rear hip.
