From the Super Mario Wiki
Baby Luigi performing a Mini-Turbo
The Mini-Turbo is a term used in the Mario Kart series, in which the player performs synchronized powersliding in order to gain a brief boost of speed. It appears in every game in the series since Mario Kart 64, and it can be activated by pushing both side directions of the D-Pad alternately when drifting. In Mario Kart 8 the Mini-Turbo can also be charged by fully steering left or right, in this case the time needed is almost doubled.
In Mario Kart: Double Dash!!, in co-op, the player who is driving must press the or button to drift while the back player must tilt the Control Stick to left and right; however, in single-player or non-co-op VS modes, each player controls both parts.
Usually, players tend to manage several consecutive mini-turbos to accelerate faster. It can also be executed on straight stretches of track, if the stretch is long enough. When this happens, the player tends to go to both sides of the track in a wavy or snake-like pattern. This is known as a "Straight-Stretch Mini-Turbo", or, more commonly, "Snaking".
In Mario Kart Wii, each vehicle is given a different Mini-Turbo rating. This affects how strong the vehicle's mini-turbo will be. The Bullet Bike has the best mini-turbo, while the Jetsetter has the worst. Also in the Wii version bikes can only receive a max of a blue Mini-Turbo while karts can receive a better orange Mini-Turbo that lasts twice as long. This is called a Super Mini-Turbo. A Super Mini-Turbo can also be performed in Mario Kart 7 and Mario Kart 8, regardless if the player is driving a kart or a bike.
Names in other languages
Turboaccelerazione in scivolata (Mario Kart DS)
Turboacceleration during a slide