Rick, you just described my stroke. My elbow drops a little on the pull back and raises slightly during the start of the forward stroke. I use an elbow drop during the follow through as well. That is why I originally referred to my stroke as a piston. However, after watching a video of myself I can clearly see the pendulum aspect to it. I guess it's a bastard hybrid of the two types. The reason I hit myself in the chest enough to be sore is probably due to the drill I'm doing. It requires a firm hit and I follow through nine inches past the CB according to the marks on my table. It appears that the left turn mark on my table happens after the tip is six inches past the CB. I have learned that at the six inch mark my arm is completely collapsed and my hand is against my chest. My hand and arm then deflect away from my body causing the part of the cue in front of my bridge to veer to the left during the last three inches of my follow through. I'm not sure if I should leave it alone or work on shortening the follow through a bit. I've hit like this for years and play fairly well so perhaps it is not an issue.
James,
What you have said about your stroke & what Jon has said is part of what upsets me a bit on this forum.
Many want to call 'every' stroke 'a pendulum stroke'. If the elbow drops how can you refer to it as a pendulum stroke when a pendulum swings from a fixed apex? So...this instructor teaches the pendulum. Does he teach your stoke or my stroke. No, probably not.
Just for an example let me offer up this hypothetical.
Someone picks out a famous pro & refers to their stroke as a pendulum stroke, when it is not. It might have one aspect in common with a pendulum stroke like the swinging up in the back stroke with the elbow still but the delivery might involve some change in the grip or the wrist or the shoulder or something because the delivery winds up being a dropped elbow & a straight extension through the ball. So... 'the pendulum stroke' gets some credit for their stroke because someone here on AZB called it a pendulum stroke, when it fact it is nothing like a true pendulum stroke except that maybe the elbow stays still in the backstroke. So now, 'everyone' wants to learn the so & so pro's 'pendulum' stroke & signs up for lessons to learn it but the so & so pro's stroke is probably not what will be taught them by the instructor because they teach a true pendulum stroke.
I hope you see my point.
I hate to say it but it is almost like the old 'bait & switch'. Not technically but still a mis-representation has occurred due to a mis-categorization of a famous pro's stroke.
As I have said before, my hand never hits my chest. I do not use a pendulum stroke & to me, in my opinion, the hand swinging up requires a perfect set up for it to work as designed. Notice I said designed as IMO it is a contrived method.
The follow through or finish after the ball is gone obviously does not effect the ball as so many want to point out. But...the follow through is effected by what happened before & during contact. In golf, a trained or experienced eye can see someone's follow through or finish & tell you what the ball flight was without ever seeing the golf ball in flight.
Can you hit the cue ball straight up table hard with a low hit on the ball & have it come back to where your tip would be if your follow through was straight, in other words without putting any english on the ball. If so, then there is probably no need to change. But... if you can not then, yes you have a flaw somewhere in your delivery. IMO the same thing goes for the up & down contact with the cue ball but it is much much less obvious & many just do not want to admit that a tip arcing up or arcing down is less precise than a tip that is moving straight & an arcing tip stroke has much less margin for error & the method of the arcing tip requires a perfect set up for it to work as designed or as I like to say contrived.
Sorry for the rant but I have not yet had my morning coffee & you hit on a touchy subject for me.
I sincerely hope that you work out your issue or find that you really don't have an issue so you can stop worrying about it & play your best.
Regards & Best Wishes,
Rick