This lecture explains how "implosion type" nuclear weaponslike the "fat man" bomb dropped over Nagasakiwork. Their core consists of a ball of fissile plutonium. When ready to detonate, conventional explosives surrounding the ball go off, forcing it to compress. This begins the chain reaction and thus the nuclear explosion.

Implosion style weapons are more efficient than gun type weapons. However, they require a greater level of precision. If part of the conventional explosion is delayed even by a split second, the ball will instead pancake and cause the weapon to fizzle.