Since a leap is not actually a mechanic, many things can cause them. A leap is simply the rear foot not maintaining contact with the ground. The cause can be a simple as a girl flexing her leg so the toe loses contact to a sever body contortion like the photo above. But in and of itself it is only a flaw in the sense that it is currently illegal in a couple of rule sets.
I would argue that the contortion is more common than the flex. There is leaping and then there is LEAPING. Fix the defect and the LEAPING issue goes away.