Basically half of the pixels have a narrow view angle, the others don't, when you activate the privacy mode, only the narrow pixels remain, so you can see the screen only looking straight.

I got this explanation for the mkbh video: https://youtu.be/nfHRMqqO578?t=141&si=iEhVrdCuLN0fkasd which illustrates it very well (2m24 if timestamp doesn't work)