In “How Butterflies Fly,” J.N. Quinn writes:
The wings of butterflies, and indeed, most insects, have a sharp edge. The sudden cut of of a butterfly’s wing pushes air much like a spoon in coffee. In the case of coffee and creamer, swirls appear around a low pressure center; butterflies use the air in the same way. Their sharp wings swirl the air above their wings making a low pressure vortex. Thus the vortex sucks the butterfly higher and allows the wings to snap back into position with less resistance.
With every beat of its wings, butterflies produce more than enough lift to keep it aloft. This over-lift causes the jerky, tumbling leaf motion associated with butterflies fight. The weird patterns butterflies move in makes catching them, for both birds and humans, more difficult.