I'm a reasonably advanced amateur, can play some of the less difficult classical sonatas and WTC Preludes and Fugues, and some Brahms and Schubert Intermezzi and Impromptus fairly well. My new teacher has been having me focus a lot on hand relaxation and position, posture, and weight. So i decided to spend at least a week just doing ultra slow practice of scales arpeggios, trills, and the pieces I'm currently learning (Bach WTC G major P&F and Brahms Intermezzo 117/3) focusing on relaxation, on finger position, wrist motion, on doing the most efficient movements (per my teacher), and keeping perfect posture. Without ever playing any music up to even 50% of tempo.
Of course it only costs a week, so I'll see whether it does any good soon enough, but I wonder if anyone has tried anything similar and if so what your experience was. I'm not so interested in descriptions of what good position, motion, etc should be as I have a good teacher to demonstrate for me, I'm just interested in whether anyone has good or bad experiences with just working on it in this way.