Overview
Trout tickling is the art of rubbing the underbelly of a trout with fingers. If done properly, the trout will go into a trance after a minute or so, and can then easily be retrieved and thrown onto the nearest bit of dry land.
History
Trout tickling has been practiced for many centuries. It is mentioned in Shakespeare's comedy Twelfth Night, where it is used as a metaphor for bamboozlement by Olivia's servant Maria, who is about to play a vengeful prank on the pompous steward, Malvolio:
The technique was a common practice used by boys, poachers and working men in times of economic stress, particularly during the 1930s Depression era. Poachers using the method required no nets, rods or lines or any other incriminating equipment if apprehended by the police or gamekeepers.
From Wikipedia (CC BY-SA 4.0).