A double transformation effect is where skin points are subjected to the action of a joint more than once, resulting in extreme, undesirable shape changes. Double transformation effects can occur if the skin object points are also being affected by a deformer that is in turn affected by the joint’s actions.

For example, if after skinning you create a cluster deformer to further control certain skin object points, then parent the cluster deformer handle to the joint that also affects the skin points, when you move the joint the points will be affected twice by the joint’s action. One way to remedy this is to organize the affected points into a set that is only affected by the cluster deformer, which remains parented to the joint.