The Somersault is a technique introduced in Sonic Adventure 2 and is used exclusively by Sonic the Hedgehog, Shadow the Hedgehog and Metal Sonic. Basically, the move involves the user attacking, while performing a somersault.
While standing still or moving at high speed, by pressing the action button, the user can perform the Somersault by flipping his body, and then proceed to attack with his legs. Once colliding, all enemies or weaker obstacles are destroyed. As shown in Sonic Adventure 2, this move can be enhanced with the right upgrade.
The Somersault first apperance was in Sonic Adventure 2 and is only usable by Sonic and Shadow (and Metal Sonic, but outside the story) and is an ideal move for close combat. If timed correctly, the Somersault can be used in combo attacks. It is however, not as fast as the Spin Dash.
Beside offensive maneuvers, the Somersault can also be used to slide underneath hazards, into small openings in walls, climb steep hills and so forth. If Sonic and Shadow receives the upgrade Flame Ring, they can perform a stronger version of the Somersault called the Fire Somersault.