Muggles' Guide to Harry Potter/Magic/Finite Incantatem

Overview
Finite Incantatem is a spell that causes easier ongoing spells and jinxes to stop.

Extended Description
Many charms, hexes, and jinxes have ongoing effects; Rictusempra, for instance, causes endless tickling, while Tarantallegra causes endless "dancing" (uncontrollable leg movement). The Finite Incantatem spell can be used to stop all these ongoing effects.

Analysis
Presumably, the wizard casting Finite Incantatem must be more powerful, possibly significantly more powerful, than the wizard casting the original spell, or Finite Incantatem would be taught as a defensive spell and would be much more widely used in dueling. As it is, we only see this spell used twice: in the Dueling Club, Snape uses it to stop all of the jinxes that have been fired by the students, and after the Battle in the Ministry, Lupin uses it to stop the Tarantallegra curse that is affecting Neville.

Some jinxes have more of a one-shot effect; Stupefy, for instance, which renders someone unconscious. Finite Incantatem would have no effect on jinxes of that nature.