Button B is a button on the Mach 5, it toggles special grip tires for traction over rough terrain (firm, icy, or unsteady ground, ocean floor, vertical mountainsides etc.). At the same time, 5,000 horsepower is distributed equally to each wheel by auxiliary engines.
In the 2008 film, the button has the same abilities as Button D which activates a bulletproof protective dome around the cockpit to protect the driver.