Now, a po on the loose in the mortal world is bad news. It turns into an evil spirit. And a po with a corpse to occupy is even worse. It becomes a hopping ghost. In documented cases, the hopping corpse is often accompanied by an entourage of monks, Taoist priests, and mourners.
Yin shock makes ghosts hop, too.
The yin, as opposed to the yang, is dark, mysterious, and usually out to make trouble. Should a fresh corpse somehow come into direct contact with a yin sort of energy, then it reacts, often becoming charged with superhuman powers. And it hops. Despite the corpse's superhuman energy, rigor mortis does set in, and the joints get stiff enough that it is forced to hop stiffly. According to some reports they have resorted to flying.