Unfortunately, drifting in Sonic Generations on the Xbox 360 is not possible.
While drifting is a common technique in other racing games, it's not a feature in Sonic Generations. The game's gameplay is focused on fast-paced platforming and speed, not on precise car handling like in traditional racing games.
Therefore, there's no specific button or control combination to execute a drift in the game.
If you're looking for a Sonic game with drift mechanics, you might want to check out Sonic & Sega All-Stars Racing or Sonic & All-Stars Racing Transformed. These games feature kart racing gameplay with drifting as a core mechanic.