When you depress the left brake pedal, in addition to braking ,the pedal sends a signal to the transmission. The signal tells a solenoid to open the pressure line that engages the clutch for the gear your machine is in, thus releasing the clutch. That allows you to speed up the engine and work your bucket and boom hydraulics faster. You can also set the transmission in reverse while that clutch is disengaged, which merely saves a little time, and makes for a smoother get away when you release the brake pedal. It's a nice feature once you get used to it.
It's also very easy to check to see if it works. With the machine in any gear, press on the brake pedal and push in a little thottle. If the machine stops and the engine revs up a bit, the clutch has disengaged. Let the brake pedal go and if the machine jerks a bit and starts moving, the system is working. Easy.