Folks:
There's a good point to this. Many of the pros don't realize that they may be compensating for something in his/her stroke. Let's take an example -- the grip hand. Since the hand is asymmetrical (i.e. you have two/three/four fingers on one side of the cue, and only one finger [the thumb] on the other), you have a symmetry problem. The hand is NOT a mechanically perfect device for delivering a cue in a straight line.
If you "look back there" and watch your grip hand when stroking the cue back and forth, you'll see a LOT of complex operations going on back there related to the fingers opening and closing, perhaps the rear fingers sliding up off the cue and sliding back onto it, etc. If I were to design a cradle to hold the cue to deliver it in a straight line, I couldn't think of a worse design than the human hand. I'd instead opt for a simple hinge that only allows forward and backward motion; it won't allow any side-to-side motion of the cue, nor involve any side-motion of the device itself (e.g. as what happens when your rear fingers slide off and back onto the cue). We've all heard of the "snatching/grabbing" the cue nastiness, right? Well, your hand is designed to do that -- its main purpose in life is gripping/grabbing, afterall! So, in order to minimize the jarring/bumping/throwing-the-cue-offline, you must make some adjustments to compensate. Those compensations may either be loosening the wrist, only letting the first two fingers grab the cue at any time, perhaps "pinching" the cue between the knuckles of the thumb and forefinger, etc.
When you put the human shooter in a stress situation -- e.g. a long shot or a shot that requires power -- sometimes the shooter is subconsciously aware that the heel of his/her hand "bumps" the cue during delivery. When the heel of the hand bumps the cue, it bumps the butt of the cue inwards towards your body. For a righthanded shooter, this means the butt of the cue gets bumped/thrown to the left (again, towards your body), which means the tip swings a bit to the right. Many pros compensate for this by aiming slightly to the left of where they actually want to shoot, and have calculated exactly how much "heel bump" throws the cue back on line, according to how much power and wrist snap they use.
Doubt me? Watch some of these pros' strokes from the overhead camera view. You'll see lining up and practice strokes that look like he/she will slightly overcut the ball. But when the final delivery stroke is applied, you'll see an OBVIOUS "hook" or "hitch" in the stroke that looks like a slight arc. For a right-handed player, you'll see the cue delivery arc to the right. This comes from the grip hand's heel bumping/pushing on that one side of the cue. Other pros address this problem by never letting his/her heel ever touch the cue (e.g. gripping the cue only with the first two fingers).
I know that pivot aimers are "Desperately Seeking Susan" when it comes to identifying pros using pivot aiming systems in videos. But like Lou, I think these efforts are roughly analogous to "when you've just walked out of the hardware store with the shiny new hammer you just bought, everything looks like a nail to you."
And like I've said before, I think folks are spending entirely too much time on this aiming system topic, as if it's the holy grail of pool greatness. Almost to the point of exclusivity! But in way, I think this topic of aiming systems is a sort of "LaBrea Tarpit," forever trapping those unwilling to trust their subconscious mind. Almost like there's a struggle for "control" by their conscious mind, and they have to forcibly keep the "aiming" part at the forefront of the conscious mind. I like to call that a "Tarpitted" pool player. Poor sap!
-Sean