It is operated by the foot brake as long as the car is on an upward gradient over x% (5% IIRC).
The car will "hold" on the slope for a couple of seconds long enough for you to put the car in gear and pull away without rolling backwards. Using the handbrake at any point will turn off the assistance.
It works through an electrically operated Servo applying the rear brake hydraulicaly.
With the EGS gearbox, you have to be careful... A quick getaway will result in wheelspin, especially if the road is wet. Perhaps the extra torque of the diesel exacerbates this. I didn't notice this with my C4 diesel EGS - it may have to do with the 'creep' function releasing and the clutch fully engaging...
It's good but I do find it annoying sometimes...there are a couple of places in Bristol where on street parking is on very steep hills.
Trying to parallel park on these hills can be a pain as once you've driven along side of the empty space and the adjacent car, you take your foot off the brake and the car just sits there until it decides to allow you to creap back or reverse back in to the space. Fine in most cases but Bristol being Bristol 2secs is a long time when you've Mr Arrogant tooting and gesturing to you that you are a [Insert Your Choice Expletive] and hurry up.
It's not a fault of the car, it's an annoyance of the feature as it did the same in the Abarth with Hill Hold.