Jay had one of his skeletons come over. He analysed the joints of the creature as well as the tendons, particularly the flexor tendons on their claws.

“Hmm…” Jay made it open and close its hand “Oh! Right!”

Jay realised that tendons don’t just hold things together, but they actually contracted and extended; he realised he would need to make the tendon in an extended state as he wrapped it around the iron bar, then contract when it was finished.