This was taught to me years ago. You need to do it daily (or most days).
As you get, even slightly, older, you will find your body will slowly contract. This drill builds and maintains your specific shoulder strength. The only issue is that if your initial mobility is poor and you have quite a stooped thoracic curve, you may need to rest your forehead on a pillow. Don’t let it hang and stress your neck!
Learn this sequence, then build in light weights