Muggles' Guide to Harry Potter/Magic/Stupefy

Overview
Stupefy is the Stunning Spell. The Stupefy jinx produces a jet of red light from the wand tip which renders its target unconscious, if it hits.

Extended Description
The Stunning Spell or Stunner is used many times in later parts of the series, once it is first shown to us. The jinx is, for obvious reasons, used whenever the author needs to completely incapacitate someone without killing them. There are other jinxes, such as the full body-bind jinx, that serve the same purpose; however, those jinxes will leave the victim conscious, able to recall what is passing while he is disabled.

The effects of the spell seem somewhat inconsistent over the course of the series. It is alternatively described as sending people flying or freezing them in place. The common factor, however, is that the target ends up knocked out. To wake up a stunned person, the Enervate charm is used.

We see the Stupefy spell used in a number of places, but most notably in an attempt to capture the caster of the Dark Mark in Harry Potter and the Goblet of Fire, to control dragons later in that same book, and in an attempt to subdue Hagrid in Harry Potter and the Order of the Phoenix. Neville attempts to use this spell in the battle at the Ministry later in that book, but his broken nose prevents him speaking the spell correctly and renders it ineffective. Harry also uses this spell in the duel at the all-night cafeteria in the Tottenham Court Road.

Analysis
Giants are mentioned as being largely immune to this spell, which apparently ricochets off them with no real effect. Hagrid, who we find out in the fourth book is half-Giant, seems to have inherited this immunity.

It is mentioned that dragons are also relatively immune to this jinx. When it is used on dragons in Harry Potter and the Goblet of Fire, we see four wizards teaming up to cast simultaneous Stun spells on one dragon.

On the Magic overview page, it is noted that there is a distinction between Charms, which do not modify the inherent nature of that against which they are cast, and Spells, which do. The Stunning spell is actually more properly a charm, because if you cast it on a person, he remains the same person, albeit now asleep. However, we assume that the term "stunning spell" is retained for the alliteration. It can also be referred to as a jinx, and is in many places, or as a hex, because it has an unwanted effect on the subject; very few people want to be rendered unconscious. It is not a curse, because the effect can be reversed without ill effect. It is uncertain where this spell would be taught, though it seems to be in very wide use; as Harry seems to be teaching it to his schoolmates in Dumbledore's Army, it is most likely one of the "defensive" spells normally taught in Defence Against the Dark Arts.