I once got curious about what petards could possibly have been... they were gunpowder explosives used for breaching castle walls. Like medieval C-4. Apparently they were a bit twitchy and more than few medieval combat engineers got hoisted.
I always assumed a petard was a rope/pulley deal to help get a weighted down knight (chain mail, etc.) up onto his horse. Now that I looked it up, I like the saying even better.
I like and use that saying myself...