Fly's got skills. The "fly aerial somersault" seen in the slow motion video is actually an escape maneuver performed when a fly is taken by surprise.
The tumble allows the little devils to regain control before buzzing off to annoy another picnicker.
Flies aren't easy to swat when they first launch off a surface, and this looks like the reason why. Their initial flight path is unpredictable and helps give them that edge right before you time your swing of death.
At least they could put the Jack down long enough to do a bit of research.