I took this with with my phone during the Dude's practice this morning. This little rubber band trick has REALLY helped him feel the difference in using different parts of his arm for his bow stroke.
To help teach little ones to play with straight bows from the elbow and not the shoulder, I put a rubber band around a foot of the shoulder rest and wound it around the right wrist. The player feels a tug when their bow goes straight down, and doesn't feel that pressure when they play a bow with their shoulder. We call the straight bows "Springy Bows" since they'll spring back for the up bow, and shoulder bows are "Swingy Bows" because they swing back and forth, not down and up.