It is often said that it is much smarter to learn from the mistakes of others than to have to suffer the consequences yourself.
Just in case you are not quite sure what a “rotator cuff” is, its a handy little muscle on the back of your shoulder that literally helps your arm to “rotate” around the shoulder socket.
For most people, this muscle is not supposed to rotate far enough to take the arms too far behind the head.
Behind the neck exercises not only pull this muscle to the edge of its natural range, but often have additional weight resistance involved that cause even more stress to the area.
If this is done repetitively over time, the muscle will often get injured. Another thing that may happen is that it will become inflammed and swell to the point where the bone around it pinches the muscle. Both conditions are chronic, painful, and should be avoided at all costs.
So, if there are any behind-the-neck movements in your current workout, you will want to find more shoulder-friendly alternatives. And if such an exercise is ever recommended to you by a friend or exercise program, ask or look for a better alternative immediately.