bold added by me:
Another interesting exercise with unbendable arm is how to break it; not literally. Firstly check your partner has unbendable arm, then place the palm of your hand under the blade of your partners. Gently drag the flesh, on the bottom of their handblade, away from your partner's shoulder (taking up the slack) and in an arc move the hand toward the shoulder. This should be quite easy, that slight tug on the partner's skin whilst moving in an arc should be able to 'break' the most unbendable of arms.
Another variation is to pull on the little finger and again move in an arc bringing the little finger to the shoulder.
I'm interested hear what results people get from trying these.
I am familiar with what you describe. I would say that these are not ways to "break" unbendable arm, but ways to illustrate if someone truly had "it" or not. There should be no slack to take out. Pulling on the little finger (as I think you are describing it) should actually provide more stability to the arm.