The bike shown is a DCT equipped model that reverts to neutral when the engine isn't running. It prevents the bike starting in gear which is good, but you can't leave the bike in gear to keep it from rolling when you park. Thus the need for a parking brake.
The mechanism seems to work like the parking brake on my Suzuki quad which has hand brakes on both sides of the handlebar. Pull the left brake lever in and use the small lever to engage a latch that locks the brake lever in place so the rear wheel can't move until the latch is moved back -- which occurs when you pull the brake lever in again.
The smaller than normal (parking) brake lever shown on the AT could actuate a mechanical rear brake caliper (as on the NC700XD) or possibly a hydraulic master cylinder working on the rear hydraulic brake caliper.
To clarify my earlier post: there are two separate calipers working on the rear brake rotor of the NC700 DCT. The top one (yellow arrow) is a regular hydraulic caliper, activated by a master cylinder and rear brake foot pedal. The bottom caliper (green arrow) is a mechanical parking brake activated via cable by a lever (of sorts) on the left side of the handlebar. The parking brake has minimal holding power - just enough to keep the bike from rolling on an incline.
I don't think the AT will have a dual calliper, doesn't appear to have in the photo's I've seen.
With ABS both brakes are used which would make an additional calliper unnecessary since both front and rear brakes will work in unison until ABS is disabled. My guess is that this will apply to that handbrake feature too.
Cant see it properly on the Africa Twin but on the NC700-750 it looks like their is one cable taking the force from the handbrake to the piston. I like how simple it seems. But I would also like to know how much is weighs compared to putting a rubber-band over the throttle and the front brake.