In the context of a planing catamaran, flying refers to allowing the windward hull to lift out of the water in order to minimize wetted surface area. These boats, especially the Hobie 16, were quite popular in the 70s and 80s.
Ha ha, we were taught to keep our optimist bathtubs halfway out of the water when sailing downwind back then. I mostly joke but no one called that flying :).
in those wonderful little square boats the reason for tipping the hull was to reduce the amount of weight it surface and dragged through the water. It was still floating, being held above the bottom by buoyancy,so definitely not flying.
The fact that they got it to work is interesting in a way because given the lack of hull length the two surfaces controlling the pitch are right next to each other.