From Sonic Retro
The Light Attack, also known as the Light Speed Attack is primarily one of Sonic the Hedgehog's techniques, useable in Sonic Adventure, Sonic Adventure 2, and Sonic Heroes. The Light Attack sees its user automatically carry out a chain of arcing spin attacks to destroy all enemies in the vicinity.
In Sonic Adventure 1 and 2, the technique is unlocked by picking up the Ancient Light upgrade from Mystic Ruins or Green Forest and Sky Rail respectively. In Adventure 1 it requires charging up in the same way as a Light Dash - a process which can be expedited by the Crystal Ring - and then just release near an enemy. Adventure 2 would be Shadow's first and only use of the ability.
The Light Attack is Sonic's special attack ability in Sonic Shuffle. Activated by drawing a "Special" card in battle, the player spins a roulette, and can deal up to 7 damage.
In Sonic Heroes, the move was one of the aftereffects of using Team Sonic's Team Blast move, Sonic Overdrive. Pressing for a short period after the Blast would make Sonic spiral into any nearby enemies. In addition, the move is automatically granted by enabling super transformation. At the cost of five Rings, Sonic may use it at any time an attackable object is in range by pressing the attack button while jumping. Tails and Knuckles are shown using the ability in Team Super's Team Blast attack.
Knuckles has a similar move in Sonic Adventure - the Maximum Heat Knuckles Attack.