[empath-nirvana]

"Don't kill your volunteers"?

edit: a more serious answer is that this is an escape trick, and generally when you do an escape trick:

1) The magician is doing the escaping 2) You _show the escape_

In this case, the volunteer is basically just shoved into the water and (presumably) drowns. The audience is left to _infer_ that there was an escape, if they choose to, or I guess they could infer that Penn and Teller are cold blooded murderers and got away with it. Either inference would be quite the good trick.