It’s a way artists hold a pencil; I don’t hold my stylus anything like that on a day-to-day basis. It’s a useful grip that I’d be constantly using if I was still working with a pencil, and more importantly, it’s a grip that forces you to learn how to draw from your shoulder, rather than resting the weight of the hand on the paper and pivoting from there. This changes everything about how you write and draw, once you get used to it.

And damn, that’s a heck of a video! It feels more to me like the pencil tip is left normal, with the line weight variance coming from very controlled variation of the pressure; you’ll notice that all the fat lines are coming when they’re pulling down. All the upstrokes are pale and thin, if they put the same pressure into those they’d probably be gouging holes in the paper. A .9mm mechanical pencil is surprisingly expressive compared to a .5 or thinner!

Edit: I just took a .7mm mechanical out and tried some similar writing; pressure can indeed do most of the line weight variety seen there.