The motion of the arms when done properly can add a great deal of speed to the swing. However if you fire the arm rotation too early then you will have all sorts of issues.
In this video with Dr. Tyler Standifird, he outlines how the proper timing of when you rotate the arms makes all the difference. The arm rotation is much faster than hip and trunk rotation so if it starts to early then the arms outrace the body and the wrist angles needed for consistent ball contact will be difficult to maintain. So check out where in the swing you should feel the arm rotation kick in.
We have a whole library of great content to help you improve your game - check out the Alta View Golf premium video library and get 7 days free access.