Muggles' Guide to Harry Potter/Magic/Tarantallegra

Overview
Tarantallegra is a jinx that causes uncontrollable "dancing".

Extended Description
Tarantallegra, the Dancing jinx, apparently causes uncontrollable jerking and twitching in the legs of the person against whom it is cast. It can apparently can be stopped by the Finite Incantatem charm. We first see this spell when it is cast against Harry by Malfoy in the Dueling Club.

It was later used in the Battle at the Ministry of Magic in Harry Potter and the Order of the Phoenix; the intent in that case was to prevent Neville from escaping.

Analysis
The Harry Potter story arc, consisting as it does of an epic struggle between good and evil, will have fights, larger battles, and perhaps even an all-out war. In order to prevent the series, ostensibly for children, from becoming excessively dark, there must be non-lethal weapons for the protagonists to use. To that end, the author has provided us with a number of disabling spells, ranging from the simple stunning, disarmament, and binding jinxes, through to those with humorous effects, like this one. It is uncertain why there would need to be so many different ways to disable, but we can surmise that the author has at least some reason for use of each variety. The Tarantallegra jinx seems to be of limited use, as the actual effect, uncontrollable motions of the legs, would have little effect on the ability of the target to cast spells; it does, however, prevent escape of the subject, as he is unable to walk. We surmise that this jinx was included as acknowledgement of Muggle fairy tales that mention magic forcing someone to dance until they die of exhaustion.