What that guy did on the video looks absolutely ridiculous from first second. Zero visual contact, too much slack (so that he doesn't have to look up), very little safety even without actual accident. When in practice there should be 100% visual contact, or at least 95% and fully covering all non-easy parts. It strains the neck massively but for that there are those cheap periscope glasses, I got mine for 10 bucks on aliexpress and they work fine enough for 10 years.
The basic technique means rope is 100% held by either hand at correct angle regardless what you need to do apart from holding it.
That said, I’ve been caught (and caught others) while half asleep on big walls with Grigris when no one could see each other.
The hardest part with a gri gri (imo) is early on when doing sport or gym belaying when there is a lot of switching between taking in and paying out slack.
[1] https://www.youtube.com/watch?v=BAxY-BBSlGc
[2] https://www.verticalendeavors.com/auto-belays-pros-and-cons/
(my climbing venue monitors and scolds you if you aren't paying attention while belaying, ymmv)