personally, think that the owner of that 159 has modified the mounting positions of the gas absorber such that it's "intermediate top of the hill" position does not exist any more
let me explain: normally those gas absorbers and the door kinematics is done such that, when opening the door, in very first part of opening sequence (lets simplify: call it opening angle) gas absorber creates push/pull force, which, together with the rest of the door mechanism tends to keep the door closed (this is safety feature). once certain "angle" position achieved, the kinematics is at the "top of the hill", meaning that it is ambivalent will it go to one or other side. if one continues increasing the "angle" (which we normally do when operating the door) gas absorber & kinematic mechanism passes "over the hill" and enables full extraction/compression.
so, in order to have gas absorber acting only one way, its attaching point either on the car, or the door has to be changed (if you are playing only with gas absorber (meaning keeping the same forces but altering the kinematic mechanism). alternative is to apply additional force somewhere else on mechanism.
try to get in contact with the owner of the YouTube video (in Croatia, according to the register plates) and ask him for details
- 146s 1.3 & 1.6 boxer (man I loved those cars!)
- 156 1.6TS (sold it only since I wanted more power)
- 159 2.2JTS SW (sold it only since I wanted more power)
- now driving 159 3.2JTS Q4 SW Ti
- now driving also Vespa GTS 300ie Super
- now driving also Aprilia Moto 6.5