Rubber Band mode provides a way to proportion the arm and leg links simultaneously. Rubber Band mode works with the Move transform rather than the Scale transform. When you move an arm or leg with Rubber Band mode turned on, both the link and its child are scaled in a single step.
Rubber-banding the upper arm rescales the upper and lower arm objects and moves the elbow link without affecting the position of the shoulder or the wrist. If you’ve spent a lot of time getting the fingers in the right place, you can reposition the elbow by rubber-banding, without affecting the hands.
Using Rubber Band to resize an arm without changing the hand position
To rubber band an arm or leg link:
As you move the arm or leg link, the hands and feet are stationary as the knees or elbows are positioned.
To adjust the biped center of mass with Rubber Band:
This button is unavailable if you are not in Figure mode or if a part of the biped other than the upper or lower arm or leg or if the center of mass is selected.
While Rubber Band mode is active, the center of mass moves independently from the rest of the biped, which remains in the same position.