Since this is based on layers, we often put one arm in front of the body, and one arm behind. But it would be useful to be able to have a hand go behind the body and then go in front. For example, a character reaches behind him for a wallet, but holds it out in front of his tummy to look at it.
Doing this would be simple if we could duplicate a layer and attach it to another animated layer, so they have identical movements and controls. Essentially, we'd have two left arms, moving exactly the same way. Hide the one in front to make it appear that the hand is going behind the body layer, or make it visible again to show the hand in front of the body layer.
So the trick is... how do we attach a layer to another so it moves in an identical fashion?