Loosely translated it means "throwing a stone and then hiding the hand that threw it".  Inherent in the act is concealment, and the expectation of going unnoticed, specially when surrounded by a crowd or mob or people.  Preferably a mob that had no involvement, directly in throwing the stone.  So the recipient of the stone has no idea which individual in the mob threw it, but he damn sure knows it came from that particular crowd.  And the stone thrower, like Mao's fish, keeps swimming along unidentified.  But the people in the mob...you can bet that they know who he was as they were standing right next to him, or heard his friends discussing the procurement of stones, or something along those lines.