To allow a bone to stretch during animation, select, go to the Hierarchy panel, click Link Info, and in the Animation Mode group, turn on Manipulation Causes Stretching. This feature lets you create bounce-and-stretch animation interactively in the viewport simply by moving the rig elements around.
Alien rig: pull the hand to stretch adjacent bones and drag the rib cage to stretch the spine
As you make the changes with Auto Key on, the animation is stored as Scale keys in the rig bone's PRS controllers. You can access these keys can in the Curve Editor, like any other keys.