i wonder if the help is more in 'rebalancing' yourself. you know. your center of gravity. often, after an injury, we tend to compensate without noticing until the injured side is much better. this puts us off balance. maybe he's helping you balance your body better, which should help piano playing immensely in that you can relax your muscles from the neck on down faster. when you exercise you practice relaxation and tenseness. you can distinguish the tenseness faster. also, you can tell when one shoulder is higher than the other by the exercises and mirrors.
i have been working my leg a lot too to get the same function back that i had before. it doesn't bother my pedalling per se as much as it bothers my sense of balance on the bench. i've been practicing pulling in my stomach, which makes the lower back up to the neck straighter, and then putting my shoulders back, and then rotating the neck just a bit. then relax and hopefully after awhile it will 'fall into place' naturally. (although it seems that you have to have A LOT of muscles in your lower back to keep the whole back in place for long hours of practice). accurate bench height also helps.
Topic: Shoulder injury,wrong advice from physiotherapist? will it hinder playing?