Tickle torture is a technique used in BDSM sensation play and can be used with consent or forced with prior consent. The "victim" cannot help but laugh and get hot from the extra circulation that laughing entails.
Besides using your fingers to tickle someone, you can also use feathers, Wartenberg wheels, and anything else you can think of.
I was undecided about the photos to include in this post. The one at the right looked so innocent compared to the oh-so-naughty one below with someone strapped to a table. ;-)