The spin dash is the super-speed maneuver utilized by most of the playable characters in the Sonic the Hedgehog
series of video games. The move involves the playable character crouching (press Down on the Control Pad) and then push the Jump button repeatedly to charge the move. Upon releasing Down the character will rocket off until friction takes over and slows him/her to a stop. This move allows your character to basically go from a total stop to near top speed without covering any ground.
Sonic the Hedgehog 2 was the game that introduced the spin dash, and both Sonic and Tails could perform it. When Knuckles became a playable character in Sonic and Knuckles he could do it as well. Other characters who can pull off the move include Amy Rose (but not in Sonic Advance) and Cream the Rabbit.
When the Sonic series moved into 3D with Sonic 3D Blast pressing Down resulted in Sonic moving southward instead of crouching. Due to this the spin dash was reassigned to simply holding down the B button. This established the precedent that in 3D games the spin dash is activated by holding down a button, whereas in 2D games Down + button is used.