In an Automatic, i thought you don't get engine braking? you NEED that for offroading, though something tells me they have some sort of system for hill descent etc?
You still get an element of engine braking just not the same neck snapper that a manual can achieve. Auto rangerovers and discoveries have been doing fine off road for donkies years.... to get the level of engine / gear braking you set the auto into 1 or 2 instead or D (drive) this stops the box from changing out of the gear you choose.
Hill descent generally uses the ABS and braking components to keep you from going above a set speed - eg it'll apply the brakes in an ABS style to keep the speed down and to avoid a skid. the idea of hill descent is to stop the rear end overtaking the front.