In this Gemora, the Zav is an engineering metaphor of indirect contact. If a Zav sits on an object, even indirectly, the object is Tamei, whereas if he moves it indirectly the object remains Tahor (even though if he moves it "directly with an intermediary" it becomes Tamei).

These two states of connection are then compared to "Regel" and Tzroros": If a carriage drawn by a horse crushes objects, even though this is indirect - as it is done by the carriage and not the body of the horse - it is called "Regel" and not "Tzroros". However, if an object was flung by the carriage and thereafter did damage, it is called "Tzroros" and not "Regel" (similar to the Zav that is not Metamei an object moved in this fashion).

Shimon Brodie