a petard is sort of like a big bomb on a stick used to blow up obstacles in a siege like gates or other such obstructions. they would approach the obstacle, usually under cover of either night or some sort of man made concealment to protect them from arrows and such, and "hoist" that big bastard on up to whatever it is they wanted to blow up. just envision that scene in saving private ryan where they blow the razor wire on the beach up with that silly bomb thing. but medieval style
EDIT- a bangalore is what the modern day version is called and very clearly it is a thing that must be hoisted
But that's hoisting the petard, not being hoisted by one. Unless you pole vault with it.