Spinning while doing a one arm hang is pretty typical when first doing these exercises. It indicates that your shoulder stabilization is not all it should/could be. Start by touching your toe lightly against something to stop the spin as you work to learn the correct muscle engagement that prevents the spin. As you gain stabilizer strength you’ll need the toe less and less.

This is analogous to hip stability. Many people are much less than 50% as strong doing a single leg squat than a normal 2 legged squat. Thats because the strength limitation for them is not in the big prime mover muscles but in the smaller stabilizer muscles of the hip that properly so the prime movers can do their work. Using two legs for the squat removes a great deal of the need for hip stability. This is why we are such advocates os the single leg box step up for mountain specific strength training.

The same goes for the shoulders. There are many small muscles around the shoulder joint whose job it is to stabilize that joint so that the bigger prime movers can flex or extend the shoulder.

Scott