You have to be standing downstream next to your buddy (victim) to pull this one off.

Reel in your line and wait for your victim to cast. Just before his drift ends, (which should be in front of you) look up river and say, "someone has one on up there." When the victim looks up to check out the action, bump his line with the tip of your pole a few times and watch him go into one of those panic hook sets. It only works one time per victim, but the look on their face lasts a long time.

I have been victimized by this. Just waiting to pull this one off on someone else.