I have a New Belgium glass that has etching on the bottom of a bicycle. I noticed that I can see bubbles "coming" from the etched bicycle on the bottom. What causes this? This was even happening towards the end of the beer.
It's called nucleation. It's a "rough" surface that causes CO2 bubbles to form as they come out of the beer. These glasses help with head retention as the head is being continually "fed" CO2. Google "nucleated pint glass" for some additional info. It's pretty cool to watch.