|Muggles' Guide to Harry Potter - Magic|
|Features||Creates a splint|
|First Appearance||Harry Potter and the Prisoner of Azkaban|
Ferula is a spell that produces a splint for a broken limb, or, presumably, another part of the body.
We see this only once: it is used in Harry Potter and the Prisoner of Azkaban to splint Ron's broken leg. It results in the appearance of bandages and a splint wrapped around the affected limb; once this spell is cast, Ron is able to put some weight on his injured leg without apparent pain.
Lupin casts this spell on Ron when they are about to leave the Shrieking Shack, saying that Madam Pomfrey is far better at repairing broken bones than he is. While Ron is able to walk after this spell is put on his broken leg, we don't have any information about whether the spell also deadened the pain, or whether it was simply immobilizing the break that decreased Ron's pain. As usual with spells, there must be some mental component; in this case, visualization of the specific splinting arrangement needed is probably necessary.