First You have to shake its head for several times.
It's easy because it is mounted in two ball bearings. That offers a long lifetime of the mechanical parts.
For shaking You can touch it in its eys, under its chin or whereever You want.
Even if you have a liquid inside which doesn't need shaking, a slow move of lupo is very good to spread the cold inside any liquid.

Then You can already tap the juice.
The position of the glass is not mechanically fixed because there are many kinds of glasses for many kinds of drinks. And the outlet of the tap is visible which makes positioning easy.

That's all!

A votre santé!