I had this function on my previous 3 cars and I really miss it, it's very convenient when you have your hands full with a child and/shopping, but primarily in the winter months where you can open/close the boot of a filthy car without getting your hands dirty/wet.
All three cars (two Audis, and a Range Rover Sport) had a pressure sensor that would stop the boot opening if it felt pressure applied. So if I realised half way through the boot opening that it might hit something as it was opening I would pull gently on the lid and on the Audi's it would stop completely, and on the Range Rover it would stop & re-close itself.