This plastic report cover is just the right size to roll into a tube.  I used velcro stickies to make it adjustable in size.  Here it is placed over the elbow of the bow arm.



This allows some natural flexibility, but the arm is essentially straight like the limb of a tree.  If you know Aikido, it’s the unbendable arm.  This extension transfers the power from the turning motion in the rest of the body into the instrument.  The hand stays relaxed. 

The opposite of this is contraction – where you’re contracting the muscles of the arm and pressing into the belly of the instrument and moving the bow with the smaller muscles of the arm.  This can lead to bursitis in the shoulder and tennis elbow.  I had both of these as a teen.  Gary Karr fixed that.