Graph Editor Interface
Below are a couple screen captures of the leftHand_FK_CTRL in the Graph Editor with the Tangents to Linear and Curves Post Infinity set to to Linear. To get the Curve to straighten out to Infinity, you need to grab the control handle and drag it until it is fairly strait. Be very careful not to move the curve! Just the handles.
Below: leftElbowForeArm_FK_CTRL with Tangents set to Spline.
Oddly enough, infinite stretch does work when I use the Move Tool. It just doesn’t work using the custom length attribute.