i think i have a similar problem to yours.as i'm taking my practice strokes on shots my elbow starts shifting slightly to my right causing a side to side upper-arm movement which causes the cue to go offline.i thought i fixed this issue about a week ago with a change in my initial alignment/back foot position but unfortunately i didn't.pidge has given you some good advice by telling you to rotate your shoulders more.i'm no expert but you can also try to bring your bridge-arm shoulder very close to your cheek (maybe even touching it) and down to the table as much as possible.this in turn will bring your shooting arm higher & further back behind the head.this might stop your elbow issues but again im not an expert
i haven't tried that last piece of advice , but it could be part of the solution. i have a very loose and bend bridge arm where most top players seem to have it fully stretched.
To give some update: i'm incorporating
several of the advices (don't go so low, turn my hips, left foot more forward, move elbow higher, turn right shoulder etc) that have been given and i'm slowly getting there. there isn't one single quick solution. So i'm going forward and improving, but the downside is, that it's almost
impossible to have it all ligned up perfectly every time, because one feet just 1 cm more to the left of forward, your hip 2° more twisted, etc and you are offline again. the rail a bit in the way and you can't place your feet the same way, etc.. I can't believe a human can align up 100% perfect every single time, but i guess that isn't needed as we don't play on russian piramid pockets.
Now i'm noticing my body automatically
subconsciously adapts to imperfections over time (i'm always a diesel in tournaments. i need 4-5 matches before i get any decent playing level). by that i mean, where in the first half hour i need to turn my back arm a little inwards after an hour it autocorrects and if i do the same pre-shot routine, i'm over -inward-ing (lol at that word) my arm.
i think there is no need to be 100% every time and as i can not see my backarm when i'm down , i needed to find a way to
feel ! if i need to do any correction.
I tried several things, i found one routine that seems to help me to very quicly see if i'm off: and that's doing some quick , loose , with long backwards movement warming up strokes (like if i would smash the sh*t out of the rack and break at 50pmh) . the further and quicker i do it, the bigger imperfections in the alignment get visible. thats when i make my slight changes in every stroke (as i don't know if my arm is outwards, or inwards, or crooked ) it can take 1 warmup or 10 warmup strokes, but if the stroke is straight at that speed, you know its 100% spot on, and you can slow the speed and distance down and concentrate on the focusing of the aiming spot. I can't do long training sessions yet because of health, but after an hour i see my ligning-up warmup strokes are go from 10 in the first balls to maybe one or two.
One sidenote i made myself. if i see that i need to correct A LOT during the warmup, it's best to stand up again and redo it all. Too big corrections make for unnatural stances, that will induce errors. so you can only adapt "so much"
I play on 3.5" pockets (with reducers), so that helps me a lot with the feedback i need for a perfect stroke and shot making. it's hard to see on 5" pockets how much off center you are
That's my experience at the moment. i will update if i have news, and i hope other who have the same problem will be helped by it too, as i know every big player has this problem. (i recently checked 3 big players , and each had a different solution. one spreads his leg as much as possible and bend his kneeds at 90° angle, in an almost karate stand (looks like a crab and that's how he got his nickname), another one bends his knees as much as possible and the other one rotates his full backarm/head, everything 30° outwards, like if he would be perfectly straight but standing on an angled flore 30° to the right. they all play very good pool, but i call it "tricks" and i know from experience "tricks" will hold you back at a certain level.