The Bushby Bit is a software switch found in Digitrax command stations. There are a number of parameters which can be set using Option Switches in a Digitrax Command Station, see your manual for a list and instructions.
When activated, it disables control of turnouts (switches) by the throttle. It is OpSw27 (default is "t" for thrown) in the command station options. It allows control of the turnouts using an attached computer. An example would be CTC (Central Traffic Control) where the train crew only operates the train, according to the signals.
Only the thottles are affected. Stationary decoders with external switches are not affected. If an operator cannot control the turnouts from his throttle, the Bushby Bit is probably the cause (OpSw27 = closed). See your user manual to find the procedure to change option switches in the command station.
More info on the Digitrax Website: Bushby Bit
In 1994, Strad Bushby developed a method to control which commands would be executed in a digital command control system by setting what became known as the “Bushby bit”. When set, commands sent to the layout from computer programs would be directed to a specific program, which reformatted and forwarded the command to the layout. He would set up a network of computers connected to his layout and DCC system. Digitax added the Bushby Bit to certain command stations to enable modellers to use computers to control signals and turnouts, leaving the engineer to run the train while a dispatcher controlled the routes. When set, the throttle cannot control turnouts.