The key to a loose arm swing is keeping your grip relaxed in the ball. Don't squeeze fingers or thumb, keep them relaxed, especially the thumb. To have a relaxed thumb in the ball your ball has to be fitted and drilled correctly. If your thumb hole feels loose it will cause you to squeeze the ball and then muscle your swing. Inserts in the thumb and finger holes can help give you a more secure fit. If you have a large webbing at the base of your thumb adding extra bevel around the top of the thumb hole will give you a deeper more secure fit.
Using Insert Tape
The use of insert tape for the thumb hole is very important. Having a good grip on the ball is a necessity to a relaxed swing. Using white textured tape on the gripping side of the hole seems to work best. It gives you something to hold onto without really having to grip tightly. A recommended fit would have your thumb exit the ball smoothly without hanging up or slipping off. This will also allow for a smooth and loose arm swing which helps greatly in repeating shots. If the ball feels like it is slipping, add a piece of tape. If you feel that it is a little snug, remove a piece. This is much easier than altering your grip.