The Insta-Shield, known as the W Spin Attack (W回転アタック) in Japan and the Twin-Spin Attack in Sonic Generations, is a special move first used by Sonic in Sonic the Hedgehog 3. Jumping then pressing the jump button again while in midair generates a brief flash that extends Sonic's attack radius and renders him temporarily invincible. This attack is useful for attacking bosses slightly out of range and spiked enemies, like Orbinauts. No character but Sonic is able to use this technique.
The move reappeared in the Sonic Advance series. It is not used in any 3D games, due to the fact the "jump, then press jump again while in midair" move was changed to the Jump Dash, or if enemies/items are nearby, the Homing Attack.
In the classic Sonic games, if Sonic jumps from a roll, he'll be unable to change his jump direction and speed. Using the move, however, makes him regain control of the jump. This also happens with Super Sonic, despite the fact that the move itself doesn't actually trigger while Super.
In the two-part Sonic the Comic story "Enter Knuckles", Sonic explains that he creates the Insta-Shield by vibrating the air around him so quickly that it creates a static effect. In the Archie comic Sonic 3 special, Sonic only uses the move once, and does not know how he managed to do so. Much later, he remembers and perfects the technique.