Donkey Kong performing a Trick.
A Trick, renamed in Mario Kart 8 as a Jump Boost, is a very useful technique introduced in Mario Kart Wii. As the name implies, they are stunts that characters can perform during jumps/air time. After the karts or bikes take to the air off a ramp or other incline, the player can shake the Wiimote (or press the D-pad on the GCN controller) to make the character do some form of stunt in the air. The higher the jump, the more intricate the stunt is, although the direction that the Wiimote is shaken in is also a factor in the stunt performed. Depending on the character, their are several tricks; Bowser and other Koopas sometimes recede into their shell ad do a spin... Mario and others do handstands, Peach and the other princesses spread their legs (yes, please!), and several other animations.
Princess Peach performing a Trick on Maple Treeway.
As well as their aesthetic value, aerial Tricks are crucial to posting good times and ratings and getting in to the top ten. Upon landing an aerial trick, the racer gets a boost comparable to a Mini-Turbo. These can be gained on even the smallest of inclines, including the shifting terrain in Bowser's Castle. This allows the player to attain much greater speeds than if (s)he simply rode through the course without performing aerial Tricks. Additionally, the more aerial tricks that a player performs during a Grand Prix, the better his/her chance of receiving a star rating. This is in addition to the time gained by performing Tricks; if two players posted the same times (assuming both got 60 points), the one who performed more aerial Tricks would receive the higher rating for the cup. They can also be done in Mario Kart 7 and Mario Kart 8.
In Mario Kart 8, when you using the Wii U GamePad, you are able to control your tricks. For example, if you were to tilt the control stick up, the character would do a forward flip, but if you were to take the control stick left, the kart would spin left. Also, the sound made when performing tricks has changed.