To use Knuckles’s All-Star Move, the player must have obtained an All-Star Item. When using it, Knuckles gets a temporary increase in speed while his theme song "Unknown from M.E." from Sonic Adventure plays in the background. How his All-Star Move is performed depends on the game. In addition, Knuckles gains a special power unique to himself, which effect also depends on the game, for the duration of the move.
Sonic & Sega All-Stars Racing
In Sonic & Sega All-Stars Racing, Knuckles' All-Star Move is referred to as Emerald Power. When activated, Knuckles pulls out the Master Emerald and uses its power to achieve flight, and he gains a green barrier around him. While blazing down the track at high speeds, he leaves a green trail behind him. In this state, Knuckles can punch the ground to create tremors that can stun and flip over other racers, but only if the Action button is hold.
Sonic & All-Stars Racing Transformed
In Sonic & All-Stars Racing Transformed, Knuckles’ All-Star Move is unnamed. When activated, Knuckles summons the Master Emerald and absorbs its power. Upon doing so, he is capable of utilizing a green energy blast (similar to the Chaos Blast) to attack surrounding racers by pressing the Weapon button. After it finishes, the Master Emerald disappears. Like all other All-Star Moves, his Land Breaker changes into plane mode if not already in said mode.