They push a bunch of air into the hive raising the pressure there and then they let it escape. To do this they need one entrance.
I think it's very cool that they can adjust the period of their ventilation to match the cavity they are in. They pulse and synchronize their fanning to match the pressure resonance when they are forced to do so by having only one entrance.... since with one entrance, continuous fanning won't work to exchange air. But I would still think that continuous fanning would be more efficient, as long as it's possible to do it. On the other hand, if you live in a hollow tree with thick wood all around, you probably don't need as much ventilation.
I use a screened bottom in a long hive, but I do have it blocked on three sides so that wind (hot or cold) doesn't overwhelm the bees control over ventilation.