Heart Gauge
A Heart Gauge is a meter that's used in the Nintendo Gamecube games, Pokémon Colosseum and Pokémon XD: Gale of Darkness. The Heart Gauge shows how much darkness is drained out of the Shadow Pokémon. The more times you would use a Shadow Pokémon in battle, the lower the darkness would get on its Heart Gauge. Once when its darkness goes all the way down, it would be possible to heal the Shadow Pokémon by restoring back to its more friendlier, lighter side. After restoring its friendship, it would now be possible to trade the former Shadow Pokémon to any of the third generation Pokémon RPG games.
The games include:
In addition, after trading to any of the third generation games, it would also be possible to trade to either another save file of Pokémon Colosseum on another Memory Card, a Pokémon XD: Gale of Darkness save file, or store it into Pokémon Box: Ruby and Sapphire. Its also possible to use the former Shadow Pokémon as a custom course in the Nintendo DS game, Pokémon Dash and also trade it into either Pokémon Diamond, or Pokémon Pearl, and even use it to battle in the Nintendo Wii game, Pokémon Battle Revolution.
Some Pokémon (like Entei and Ho-oh) were previously unavaible before when Pokémon Colosseum was released in the U.S. in March of 2004. Most of the Shadow Pokémon are from both the first and the second generations.
The Lugia factor
In XD, there's a Lugia called "XD 001". Unlike all of the other Shadow Pokémon, you would have to have nine perfect tempos playing in order to restore Lugia's former self. That way, you can trade it, just like any other former Shadow Pokémon. Before restoring, you can't use Lugia to battle in order to restore. You also can't use any scents (like Joy Scent, Excite Scent, or even Vivid Scent) in order to restore Lugia's friendship. There's only one Lugia in that game, and its also one of the only few places where you can get it in order to get it recorded on to the PokéDex.