The Spindash is a move that can be used in most Sonic the Hedgehog games.
It is usually activated by holding down on the d-pad, and pressing a fascia button, for example, any button on the Sega Megadrive control pad, B on the Game Boy Advance, etc.
The Spindash is a move where Sonic curls himself up into a high speed spinning ball, and when the player releases the fascia button, Sonic will dash forward in the spinning ball. The move can be used to swiftly gain acces to higher points in the game, through use of quarter-pipe style slopes, to attack enemies, to gain speed or to just propel oneself forward rapidly.
The move was first seen in Sonic the Hedgehog for the Sega Master System.