Priori Incantatem, or the Reverse Spell Effect, is used to detect the spells cast by a wand. The spells cast by the wand will emerge in smoky or ghost-like replicas in reverse order, with the latest spell emerging first. It is first encountered in Goblet of Fire when the house elf Winky is found holding Harry's wand. This spell is used to reveal that it was indeed Harry's wand that cast the Dark Mark. In Harry Potter and the Half Blood Prince, it is revealed that the teenage Voldemort murdered his father and grandparents using his uncle Morfin's wand, knowing that, when examined, the wand would incriminate Morfin as the murderer. In Deathly Hallows, Harry feared that a Prior Incantato spell would be used on Hermione's wand after the Death Eaters had a hold of it. This would reveal that she had accidentally broken his holly-and-phoenix-feather wand (with her wand) when a curse misfired and they both narrowly escaped Voldemort earlier. As a result, the protection of the shared cores was lost and, worse still, this would now be made known to Voldemort. During the final duel between Harry and Voldemort, the latter mentions that he knows that the holly and phoenix wand is destroyed, implying that the Prior Incantato had indeed been performed on Hermione's wand, as they had feared.

Forcing two wands that share the source of their cores to do battle can also cause a more potent form of Priori Incantatem. The tips of the two wands will connect, forming a thick golden "thread" of energy, and the two wands' masters fight a battle of wills. The loser's wand will regurgitate shadows of spells that it has cast in reverse order. This phenomenon occurred during the duel between Harry and Voldemort at the end of Goblet of Fire. Their simultaneous spells (Harry and Voldemort cast "Expelliarmus" and "Avada Kedavra" respectively) triggered the threads, and as Voldemort lost the battle of wills, his wand regurgitated, in reverse order, shades of the spells he had cast with it – screams of pain from torturing various victims, echoes of the people his wand had murdered: Cedric Diggory; Frank Bryce; Bertha Jorkins; as well as Harry's parents. Dumbledore later revealed to Harry that his and Voldemort's wands both shared a tail feather given by Dumbledore's pet phoenix, Fawkes. Harry was previously informed by Mr Ollivander that the holly wand that "chose" Harry was the "brother" of the yew wand that gave him the lightning-shaped scar on his forehead, although the significance of this was not discussed at that time.