Instapundit's review: "Devastating."
In medieval and Renaissance siege warfare, a common tactic was to dig a shallow trench close to the enemy gate, and then erect a small hoisting engine that would lift the lit petard out of the trench, swing it up, out, and over to the gate, where it would detonate and hopefully breach the gate. It was not impossible, however, that this procedure would go awry, and the engineer lighting the bomb could be snagged in the ropes and lifted out with the petard and consequently blown up. Alternately, and perhaps a more likely scenario, if the petard were to detonate prematurely due to a faulty or short slow match, the engineer would be lifted or 'hoist' by the explosion.
Thus to be 'hoist with his own petar' is to be caught up and destroyed by his own plot. Hamlet's actual meaning is "cause the bomb maker to be blown up with his own bomb", metaphorically turning the tables on Claudius, whose messengers are killed instead of Hamlet....