This thing is one of the rare items that does exactly what it is advertised to do. It gives you a tool that enables you to stretch the internal and external rotators of the shoulder in a way that you cannot do by yourself.
If you have tight shoulders with reduced ROM in either internal or external rotation, or both, this is an excellent and easy to use device to help you stretch them, consistently and progressively. This may be the best way to improve that flexiblity without a manual therapist working on you.
Lots of vids on the website