Re: johnson 5.5 lower unit
Spring loaded in a 2-piece case. Expands lengthwise when prop hits an obstruction, causing friction so the motor stops spinning. It's designed to save the prop/gears when you hit a rock. They were used when props didn't have rubber-hubs which perform the same purpose. If yours is broken, it's likely expanded too far for normal operation. Again, search this forum for lots of info.