In Scotland the technique is more often called "guddling" or sometimes "ginniling"

Trout tickling is the art of rubbing the underbelly of a trout using fingers. If done properly, the trout will go into a trance-like state after a minute or so, and can then easily be thrown onto the nearest bit of dry land.

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:

  Close, in the name of jesting! Lie thou there,
  for here comes the trout that must be caught with tickling.

LINK (via: Wikipedia)