From Sonic Retro
While cornering at high speed, Sonic leans to the side so his fingers are touching the ground; a technique which can give the hedgehog a much tighter turning circle. In Generations, Sonic will roll tightly instead of holding his hand out. It is activated by holding or / while rounding a bend in Unleashed, or either / or / in Generations.
Drifting is particularly important for gaining a good time in the Savannah Citadel acts, where scrubgrass beside the trails has the potential to slow Sonic down to a crawl if the player's cornering is insufficiently precise to remain on the main causeway. Savannah Citadel Day Act 3 features hairpin bends beside bottomless pits, making good use of the Drift utterly essential for acquiring an S-rank (or even staying alive).
The Speed Drift was one of the most difficult moves for players to get to grips with in Sonic Unleashed. Success is generally improved by turning Sonic in the appropriate direction before you hold down to Drift. This significantly tightens the turning circle once Sonic actually starts using the Drift technique.
Sonic Colours restricts the move to certain sections instead of being a move he can perform at will.