In Deathly hallows, Harry fears that by using Priori Incantatem, the Death Eaters could work out that his wand was broken:
“Priori Incantatem,” said Harry. “We left your wand and the blackthorn wand at the Malfoys’, Hermione. If they examine them properly, make them re-create the spells they’ve cast lately, they’ll see that yours broke mine, they’ll see that you tried and failed to mend it, and they’ll realize that I’ve been using the blackthorn one ever since.”
Deathly Hallows, Chapter 24 - The Wandmaker
Almost everything I know about this phenomenon is through Dumbledore's explanation in Goblet of Fire:
“The Reverse Spell effect?” said Sirius sharply.
“Exactly,” said Dumbledore. “Harry’s wand and Voldemort’s wand share cores. Each of them contains a feather from the tail of the same phoenix. This phoenix, in fact,” he added, and he pointed at the scarlet-and-gold bird, perching peacefully on Harry’s knee.
“So what happens when a wand meets its brother?” said Sirius.
“They will not work properly against each other,” said Dumbledore. “If, however, the owners of the wands force the wands to do battle . . . a very rare effect will take place. One of the wands will force the other to regurgitate spells it has performed — in reverse. The most recent first . . . and then those which preceded it. . . .”
Goblet Of Fire, Chapter 36 - The Parting of the Ways
So we know that the effect requires that both wands share cores (from the same animal, not just species; this answer shows what they could be). This will cause them to not work properly against each other and connect. The regurgitation starts after one owner has won the battle. However, I find it hard to comprehend how the Malfoys could find the right counterpart to Hermione's wand in the first place. Oh, and Dumbledore calls this a very rare effect.
- Is the effect easily re-creatable?
- Also, does another occurrence of this effect exist in-universe? If yes, perhaps it would come in handy.