Amos Diggory uses Prior Incantato to find out whether Harry's wand was used to conjure the Dark Mark. Placing his wand tip to tip against Harry's wand and saying the spell, Amos causes a shadow of the Dark Mark to erupt from where the two wands meet, showing that this was the last spell cast with Harry's wand.
Prior Incantato is the incantation of the spell Priori Incantatem. Dumbledore reveals that because Harry and Voldemort's wands share cores, each containing a feather from the tail of the same phoenix, Fawkes, they will not work properly against each other. If the owners of the wands force the wands to do battle, a rare effect will take place. One of the wands will force the other the regurgitate spells it has performed in reverse; the most recent spell first, and then those which preceded it. A kind of reverse echo appears; in the case of Voldemort's wand, a shadow of the living Cedric was the first to emerge from the wand.