HDC works by bringing the Landy down to a safe, preset speed (it's set in the electronics!) without locking the wheels. You can test it on a flat road by say accelerating in 3rd (still in low box or else it doesn't work!) then shutting off - the Landy will then apply the ABS to bring the speed back down rather than just letting engine braking do it. Of course, on a steep slope, engine braking alone might not be enough to prevent the Disco from picking up speed on the overrun, hence it applies the brakes for you. Similarly on a slippery slope, engine braking may cause a wheel to lock up, so the HDC will prevent this in order that you'll have directional control (in theory!), whilst it slows the vehicle down.