The Somersault was first introduced in Sonic Adventure 2, and also appears in Sonic Advance and Sonic Advance 2. When playing as Sonic or Shadow, if you press 
, they will do a somersault that damages enemies. If is pressed mid-somersault, you can do a combo. Somersaults can be used to get under small openings in doors, to climb steep hills, etc. In Sonic Adventure 2 Sonic and Shadow can get a level-up item called the Flame Ring, which enables the hedgehogs to break steel containers with a "Fire Somersault".
In Sonic Advance 1, the move is a little more involved. Pressing while moving forward first sends Sonic into a loose spin roll (which can still smash enemies or monitors); pressing again while in this roll sends him hands-over-feet in a somersault, and pressing again deploys the Skid Attack, where Sonic can slide feet-first into an opponent. Alternatively, pressing 
while in the somersault makes Sonic Backflip into the air.
Sonic Advance 2 dispenses with the three-step approach; pressing while moving forwards sends Sonic straight head-over-heels into a Skid Attack.
...finally skid attack...
