playbill-love said: "There were quite a few empty seats in the mezzanine and very few people coming down from the balcony on the way out, so if rush is based on availability it seems like you should have a good chance."
I don't believe that is what it means that rush is based on availability means. They usually only give a certain number of tickets away for a given show.
However, if a show is selling well, and the house sells out, then there wouldn't be a rush, since there is no availability.
It means they don't hold seats specifically for rush, not that they will sell as many rush seats as there are unsold seats.