Belt Pulley. How get relative velocity??

Hi everybody!

I'm trying to understand how belt pulley from Mathworks work. I have this simple sample:

In this case. I'm giving the velocity of A. And there is when appear my first question:

why the magnitude of velocity of B, is the same of A?

According to the theory of belt drive, the ratio between both velocities is this:

But the "Belt Pulley" description of Mathworks, said that both will have the same magnitude but different direction.

Anyway, I could assume that they are not consider the elastic of the band, but I don't have any clue of how they get the angular velocity of the pulley to get then the relative velocity. At some point they need to know the elastic of the material for get the relative movement between the belt and the pulley.

Can anyone help me?