What is Arm Drag?
To me, arm drag is a failure of the arm to keep pace with the shoulder rotation. Now, that’s a little bit misleading because it’s usually not the arm’s fault - it’s usually the fault of other parts of the body and a flaw in the mechanics earlier in the delivery. Tom House says there’s no such thing as a late arm - only early shoulders.
Arm drag often happens when the shoulders open up early. All the energy being transferred up the kinetic chain gets wasted because there is not smooth transfer from the shoulders to the arm. As a result, the pitcher ends up throwing with just his arm. Pitches usually end up going up and in or down and away (due to over-correction). Doing this habitually can lead to more wear and tear on the arm (the shoulder especially).