Hey there! The one-pedal driving mode on the Polestar 2 is pretty cool!
It primarily uses regenerative braking to slow down the car, but there are certain conditions when the mechanical brakes will go on:
On quick stops - whenever you need to stop faster than normal, like when you are approaching an intersection. In that moment the regen braking isn't enough to slow you down, so the mechanical brakes go on.
Or on low battery - when you have lower energy, the regen braking is limited. The mechanical brakes will take over to bring more stopping power.
You can tell when those mechanical brakes are in action when you feel a distinct difference in braking force and a change in pedal feel. If you have driven a car with a traditional braking system, then it is easy to notice the difference. Does it help?