GM Service Manual Online
For 1990-2009 cars only

When the ignition is in the run position, The Auto Trans Shift Lock Control (ATSLC) system prevents the driver from shifting out of the PARK position without simultaneously depressing the brake pedal. When the ATSLC is energized, the actuator locks the shift control lever.

When the brake pedal is depressed, the actuator de-energizes and releases the shift control lever and the driver is able to shift into the gear selected.