While holding the B button, Bowser continuously exhales a stream of fire in front of him, which gradually decreases in length over time. In Brawl, the flames can also be angled up and down, and they will never completely recede, as they will in Melee, where Bowser will eventually begin to exhale smoke instead.
The move's long duration is useful for staving off opponents and preventing their return to the stage. Unfortunately, an opponent using DI can escape the flames fairly easily in most directions. The fire will flow off of floors and walls, and can actually split in two directions, if the angle is perpendicular.
This special attack is shared by Giga Bowser, both in his boss appearance in Melee and in his playable appearance in Brawl. There is no time limit to how long the attack can be held in his Melee incarnation.
 Flame cancel
In NTSC versions 1.0 and 1.1 of Melee, Bowser can cancel the startup lag of this move if he lands during its initial frames, in a manner similar to the space animals' short hop laser. This technique is not possible in the NTSC 1.2 and PAL versions.
Bowser's ability to breathe fire has been demonstrated in many games he's appeared in, including his debut in Super Mario Bros., where he would shoot fireballs at Mario, but it was not until Super Mario 64 that he was shown to breathe continuous streams of fire.