Could someone please explain this one to me?

> If you really want a glass of water at a restaurant, always order that first. As you do this, look the server in the eyes and nod.

I don't understand it at all. Is is to make sure you get the water first, and not with the other dishes? Is it to test how the waiter would react, and whether you want to order other things?

Why that weird ritual with the eyes, what is it supposed to mean?