In the water salutations we work with crossed legs. Parallel legs can hold only so much stamina, so we create a "bolt" by crossing-over, substantiating the sacral plate. With so many repetitions of our sun salutations, and so much demand on the legs and lower back, crossing over is like braiding the legs, enabling one's lower body to hold up better, developing stamina and stability rather than unraveling or leaking out energy.