Muggles' Guide to Harry Potter/Magic/Locomotor Mortis

Overview
Locomotor Mortis is the "leg-locker" curse. Similar in effect to the body bind, this spell causes only the legs to be bound together.

Extended Description
We see this spell used only a very few times. Draco Malfoy uses it on Neville in Harry Potter and the Philosopher's Stone, though that is done off-stage and we only hear about it from Neville after he manages to hop into the Gryffindor Common Room. Harry Potter attempts to use it against Draco Malfoy in Harry Potter and the Half-Blood Prince during their duel in the bathroom, but misses, shattering a cistern.

Analysis
This spell is of limited utility as it leaves the target's wand arm free, so that he can either free himself or retaliate. It is likely to be useful only on a disarmed opponent or one without developed magical abilities. As such, it is probably a classic first-year sort of jinx, one that impedes the opponent but does not harm him particularly.