There's no point whatsoever in doing this. When you are close to ovulating, your cervix opens to allow the sperm through. When your partner ejaculates, it's done at such a speed that all the good sperm will go straight into your cervix and end up in tiny pockets so they can't fall out.
What leaks out of you after sex is seminal fluid, not the actual sperm itself. So there's no point holding legs in the air. There's also no point holding in your wee if you need to wee after sex (all it does it risk water infections).