If a character can use the ability, then he/she is able to run at extremely fast speeds, usually leaving afterimages of the character. It may have different effects based on the game it appears in.
Sonic Advance 2
In Sonic Advance 2, the character is simply able to run significantly faster with increased acceleration while Boost Mode is in effect. The more Rings the player has obtained, the faster the character can enter the mode. If the character slows down or is hit by an enemy then the boost is automatically ended. Sonic can also engage this manually if he performs a failed Homing Attack and hits the ground running. For the entire duration of the move, the character can perform a Boost Attack which varies depending on the character.
Sonic Advance 3
In Sonic Advance 3, is basically the same as in Sonic Advance 2, but without the Boost Attacks. However, only speed-based teams are able to use it. If Sonic is the character's partner, he/she can manually engage it via Tag Action. However, if the character is partnered with Cream (except for Amy) and performs a failed Homing Attack and hits the ground running, then he will automatically enter Boost Mode.
Sonic Rush series
In Sonic Rush and Sonic Rush Adventure, both Sonic and Blaze can enter the Boost Mode. In these games, there are two ways to enter the Boost Mode. The first one is by running at a constant level of high speed for a certain period of time, and the other is by activating and then disengaging the Super Boost/Fire Boost while moving; in the case of the latter, when the Super Boost/Fire Boost ends, either on its own or at the player's command, the player will be left in Boost Mode should he/she continue to move forward.
Sonic Colors (DS)
The Boost Mode reappeared in the Nintendo DS version of Sonic Colors. In this game, its performance and mechanics are the same as the ones presented in the Sonic Rush series. When entering the Boost Mode through running in this game, Sonic briefly gives a of a rainbow-colored aura in front of him. However, when he's boosting over water, it becomes a blue aura before changing back to rainbow-colored once Sonic is out of the water.