astrophysics - Intensity mapping of the 21 cm line

I'm currently reading Astrophysics for physicists by A.R. Choudhuri. Section 6.5 of his book he presents the following plot:

and tries to explain how it was generated. I'm having a hard time understanding how we can differentiate between the different velocities. As far as I understand we only measure the intensity $I(l,0)$ along the line of sight for some $l$. So I would expect that we are able to plot $I(l,0) \text{ vs. } l$, but I don't see how we can actually know how the velocity distribution of the ISM along the line of sight is.

In his book, Choudhuri talks shortly about $$v_R = (\omega-\omega_0)R_0\sin l,$$ where $\omega_0$ and $R_0$ are the angular speed and the radius of our sun to the galactic center. I think he uses this to somehow explain how we can actually measure $I(l,0,v_R)$ instead of $I(l,0)$ but I just couldn't follow his arguments.