Look, I just went out, enabled it on my car and went for a test drive.
Little green car on a hill top left of the dash, no other warning lights.
Hold the brake on a hill and let go? you roll down the hill.
Holding the brake, press in the clutch, little green car starts to blink fast, you are now held on the hill, release the brake, select first, and off you go.
I don't know how much the the level, or lack thereof, of the car affect the system if you enable it while parked on a hill, I enabled mine in my parking spot, on level ground.