Animation noob here--noticed there's some funky bones in this model exported from WoW.
In the screens, it shows the vertex weights for this example. (Also, it's parented to the elbow, and has no children of its own. Might not be clear from the shot.) Obviously these weights intersect with the weights given to the surrounding bones. I then looked at a sample animation, and noticed that this bone only animates displacement. (They use the same technique in many spots; upper arms, legs, some around his hunchback.)
What's the point of that? Is it a "hack" for solving weird pinching/volume loss? Would one use these "displacement bones" in a higher poly asset?