Rubber Band mode provides a way to proportion the arm and leg links simultaneously, by moving the link with the Move transform, instead of using scale. Rubber Band mode scales both the link and its child in a single step.
This is particularly useful when fitting a biped to a skin prior to applying the Physique modifier. For example, 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
Turn on Figure mode.
Select the arm or leg link you want to rubber band.
Turn on the Move transform.
On the Motion panel > General rollout, turn on Rubber Band mode.
This button is grayed out if you are not in Figure mode, or if a part of the biped other than the upper or lower arm or leg, or the center of mass is selected.
Move the selected arm or leg link.
As you move the link, it stretches the link and its child.