Their thick skin and blubber don’t allow the gases to diffuse out very well, and sometimes the expanding corpse seals up its own holes to compound the problem. Puncturing the side of the body allows the gases to leak out slowly, thus avoiding an explosion. Ironically, it’s probably human interaction with the bodies that causes them to explode in the first place. As people push on the bodies and try to move them, they can rip holes in the skin that act like stress points in a faulty structure. One spot of weakness makes the entire body more likely to explode at that site. And if it’s warm out—like it is in New Zealand right now—the gases expand, increasing the pressure.