iternabe Then at the end of the video, she explained that a hunched postures will cause that problem, and sitting straight with shoulders pulled back solves it. I believe she is correct. On the other hand, I know I have a hunched back problem half a century in the making, so it could be really hard for me to correct.
I must have linked the incorrect video! There's one where she starts out describing the "modern" back position of being slightly hunched over with elbows typically out, and explains how that allows you to play with more power, but less fine control (for playing quickly and quietly), in comparison to the straight-backed, elbows-in position. I will look for the right one when I have more time.
Re: hunched back position - mine was 40-45 years in the making. I'd always had various postural and gait problems, but no one ever pointed out the issues that my hunched back caused until I saw an upper extremity doctor for pain and limited movement in my shoulders. He said that I needed shoulder debridement to get rid of two bone spurs, but that a bigger issue was my hunch, which would require physical therapy. (I've seen physical therapists since I was 9, and no one ever thought my hunch was anything more than pure laziness.) The right ones quickly realized that because of the lifetime of hunching, my chest muscles and tendons were far too tight, and I had no idea of how to engage a specific pair of muscles in my back whose job is to pull the shoulder blades back and down.
In only a few weeks I could trigger the correct back muscles, and honestly, it's sort of changed my life. I don't want to sound dramatic, but the hunching restricted my lungs, and my shoulders constantly being too forward meant that my entire skeleton couldn't just rest in 'neutral' when I was standing or sitting, which explains a ton about my physical issues from childhood onwards.
Anyhow, you might want to look up "A" and "T" back exercises. If you can't bring your shoulders back on your own, you'll probably have to find a physical therapist who really knows what they're doing to tap on the right parts until you know what you're doing, but honestly, I think the long-term effects on one's health are so significant that it's worth the time and money, if at all possible.