According to ChatGPT it's the skill of taking exactly one sip such that when you put the Guiness glass down the new beer level ends up precisely through the horizontal middle of the "G".

According to LadBible the actual trick was drinking a pint of Guinness while standing on his head.

https://www.ladbible.com/entertainment/netflix/netflix-execu...

Nor am I but I found this. https://www.youtube.com/watch?v=fL35y1nTkxQ

It seems to be a challenge where you drink a specific amount from a pint in one try such that the bottom of the foam splits the Guinness logo. It's not even a huge amount. It seems to less than 20% of the pint eyeballing it.