It is very effective. What you have to remember is that you have to get your opponet off-balanced. If doing a right handed arm spin, grab his right arm tightly. Take a few steps backwards before spinning in. It is even more effective if you push him back and when he pushes you (and he will) use that momentum to help with your pull. Once you spin in, keep your elbow in his arm pit and try to put your thumb in his ear. You will never put your thumb in his ear, but it is a good way to keep yourself tight on his arm.
Originally Posted by Laynerz
That was one of my favorite throws in wrestling and I know use it very effectively in judo. If you can't get it to work, it is probably because he isn't off balanced enough. You want to get him leaning forward on his toes before you spin in. If not, he will just throw his hip into you which will stop the throw. Keep trying it, it does work.