Yes, it pushes on the latch that holds the slider open on a hill. The latch is released by operating the door handle when it is fully open. They are available as spares, I once spent ages making one out of a clothes peg spring. It does not work as well as it did at first, especially in Bristol, and having your leg whacked by the slider suddenly is not fun, although if you are tuned into the rumble as it starts off you can get clear...
The spring to hold the door open is p/n 211 843 755 https://www.alanhschofield.com/ourp...ing-Door-Mechanism-Spring-6879-211843755.html