Arm drag is due entirely to 2 things. The first has to do with arm action. Pitchers generally do one of two things. They either lead with their elbow or lead with their hand. The difference between the two can have a drastic effect on stress to the joints. A pitcher who leads with his elbow will cause his arm to be whipped around because it takes too long of a path to try to get in the power position which generally creates both a timing issue and even a shorter arm action in some cases. A pitcher who leads with his hand ( like a conductor leading in music) has a much cleaner action and also has a shorter path to get to the high power position, thus eliminating the violent whipping of the elbow joint. f their is an arm action issue, it may not matter at all what the hip is doing or when it is opening up because thearm action is not right, too slow, etc. which creates the whipping action regardless of staying closed little or lots.
The other element to arm drag is timing. A pitcher can have clean arm action and still be late because he may wait too long to break his hands or be too slow in his arm action.
We learned years ago that arm timing has almost nothing to do with hip timing and action but rather what the shoulders or upper body is doing. It was explained and shown to us in a simple throwing drill. Stand straight across from your throwing partner and without any initial hip rotation, keeping them square with your partner, and then start your arm action, swinging the arm in a nice clean arc while also turning your shoulders until you get maximum separation from your hips and then stop in that power position and note that you can get separation more than 90 degrees between your hips and shoulder angle. Then do the drill again but this time dont hesitate and throw the ball. This shows that the hips can and will be open when still in the power position. What happens a lot with arm timing in this drill is those who are late will never reach the high power position before the "shoulders" start rotating the arm into release even though his hips are open just like the kid next to him with the right arm action and timing. Its a simple drill and really shows arm action and timing issues.