This is how bmw explain it:
Quote:
Electromechanical Parking Brake.
Starting off on a slope, in stop-and-go traffic or whenever the engine is turned off: the Electromechanical Parking Brake ensures your BMW stays comfortably and reliably at a standstill whenever required.
Working together with Dynamic Stability Control (DSC), the Electromechanical Parking Brake controls all braking processes whenever the vehicle is not moving. If the engine is running, it operates hydraulically via the DSC brake system. If the engine is switched off, electromechanical brake cables support the function of the conventional handbrake.
The Electromechanical Parking Brake can be activated by pressing a button on the instrument panel or, depending on your BMW model, on the centre console. It also offers an autostop and hillhold function.
Autostop automatically activates the parking brake whenever the vehicle comes to a halt; it disengages the moment you touch the accelerator. In vehicles with automatic transmission, it is no longer necessary to keep the brake pedal applied to prevent vehicle creep when stopping in gear, such as in stop-and-go traffic or at traffic lights. This makes city driving significantly more comfortable.
Hillhold automatically applies the parking brake whenever your BMW stops on a slope, preventing unwanted rolling, and disengages it when you start off again.
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http://www.bmw.com/com/en/insights/t...ing_brake.html
I'm a fan of these "auto-release on gas" button type parking brakes - it's a little aggressive in terms of when it applies via the auto-hold but that's just something to get used to.