The handbrake, might be a staging brake...without knowing what kind of staging set-up the car has. That may have been the activation switch for the 2-step most likely. I have also seen a pressure guage inline with a staging brake, this switch may have also been to turn on a light at a preset psi...maybe 300 to let you know when you had enough pressure in the brake system to start to preload against the clutch to be consistant, and not guess every time. I use my e-brake so I don't know what psi is needed to hold a car back on the line.
Kevin