In Colosseum, Shadow Pokémon would randomly go into a mode called Hyper Mode which increased the Critical Hit ratio of the Pokémon. However XD has a totally different mode than Hyper Mode, it is called Reverse Mode and has a few differences

Reverse Mode is different to Hyper Mode as it does a few things that Hyper Mode doesn't. Below is a list of Reverse Mode effects: The Pokémon still attacks the turn it goes into Reverse Mode The Pokémon damages itself each turn The Pokémon only attacks with Shadow Attacks, if Non-Shadow Attacks are used then the Pokémon may not obey The only way a Pokémon can get out of Reverse Mode is by the trainer Calling to it in Battle or using one of the Scent's

Thats basically it for Reverse Mode, a flaw in the Shadow Pokémon, but it adds to the challenge for the battles in XD