From Bulbapedia, the community-driven Pokémon encyclopedia.
The Aura Reader is a piece of equipment found only in Pokémon XD: Gale of Darkness. It is a scanner worn on the ear with a screen which extends in front of the eye. It is an attachment to the Snag Machine.
The Aura Reader does not play any role in snagging itself, but it plays a critical role beforehand. In a Pokémon battle, when the opponent sends out a Pokémon, the Aura Reader springs to life and identifies whether or not it has the dark aura of a Shadow Pokémon. In Pokémon Colosseum, there is no Aura Reader because Rui can see the auras and identify the Shadow Pokémon on sight. Once the Shadow Pokémon is identified by the Aura Reader, it can be safely snagged without setting off the Snag Machine's Alarm system.