Essential Powershift. The car rolls even though the gear is engaged.

What is your question?

The car is a powershift (automatic) and i don't understand why would the car roll either forward or backwards when the gears are engaged? Even though the car is in D mode, one almost have to put the hand brakes on when you are on a steep hill to avoid the car rolling back. This does not happen when i drive my Mercedes Benz. Please advise?