The brake switch provides a signal when the brake pedal is pressed. It tells the vehicle computer to apply or release the Torque Converting Clutch (TCC) as appropriate.
As soon as the brake pedal is pressed, the control module detects a preset voltage signal, which is sent to the Engine Control Module (ECM) as serial data on the status of the brake lights. The control module receives a signal from the switch, indicating that the driver has depressed the brake pedal. Typically, the switch is always closed and receives voltage through a fused circuit. For this reason, the control module observes the ignition voltage when the brake pedal is depressed.
The DTC code P0703 OBDII is set when the control module does not get the brake-pedal shift signal so the Check Engine light will illuminate. This is a high/low signal, and it is when the computer gets or does not get the voltage signal respectively.