Kusuguri (tickling) (くすぐり)
Kusuguri (tickling) is to give 'tickling' sensation by stimulating skin surface and 'make someone laugh.'
Mechanism for tickling
The body parts that you feel tickling sensation are generally the parts where arteries are close to the skin, such as surrounding of ears, crest of neck, armpit, back of the hand, base of thigh, ham, and dorsum and sole of foot. These parts of the body are 'dangerous parts', which cause excessive bleeding in case of injury. Therefore, autonomic nerves are centered around these parts of the body, so people are especially sensitive to outside stimuli. The cerebellum is closely related with the autonomic nerves and controls the prediction of the stimuli for these dangerous parts and feeling of them. Accordingly, even if you tickle these parts by yourselves, the stimuli is predictable and you don't have uncomfortable feeling because the cerebellum controls the feeling. However, if others tickle you, the cerebellum cannot predict it, so the feeling is not controllable and the brain will be thrown into confusion. The uncomfortable feeling is 'tickling' feeling, and an excessive response of autonomic nerves, which try to avoid the illusionary state of 'a potential crisis of life', causes 'laugh'.
Sexual tickling
Thus people instinctively prevent others from touching their 'dangerous parts,' but on the contrary, allowing it would be a token of enhanced confidence or affection. Therefore, tickling with permission might be a sexual pleasure in some cases.
For details, please refer to the 'Tickling Fetishism'.
Various kinds of tickling
Tickling could be used as a cure for hiccups.
Formerly, tickling was executed as one of the tortures.
Tickling as traditional performing arts
In the traditional performing arts, performers sometimes go out of their ways to make the audience laugh by off-topic dajare (pun) and inside jokes, and that is called as 'kusuguri'. Especially, in Kabuki and Classical Rakugo (classical comic story-telling), unimportant words in the original lines are sometimes replaced with today's related 'person of the day' or 'topic of the day', and the audience cannot help grinning at such a scene. Although there is an exception like Sanpei HAYASHIYA, this kind of technique is basically not in heavy usage and is used only once per act/story, if ever. Moreover, performers play this at the scene where the audience cannot expect, so kusuguri can be a technique giving an accent to the play.