sorry brian i hate to start an argument and i hope i dont, but i believe the arms CAN still be goin back whilst the lower body starts. this is not however a manipulation on behalf of the golfer, which i totally agree with. i believe this is a natural reaction of the brain. as the arms swing back, the Central Nervous System (CNS) calculates that the arms are goin to swing back at the ball with hopefully more power and speed then they did goin back.
the body's natural reaction is to clear the left hip. the faster the player intends to make his transition, the earlier the CNS will clear the left hip. in players like bubba watson whos transition is immensly fast, the lower body needs more lead time than most.
if an average player goes out and tries to replicate this however, by manipulating the hips forward whilst the arms are being swung back, then yes, some very strange shots will be hit. it is simply because there swings and transitions are not fast enough to merit this move.
i also agree the momentum of the club can also create the illusion of this move. but i still think that it is possible for the arms to physically be moving back whilst the lower body starts forward