I like Dan's answer, but oddly enough I visualize different poses when I read the OP's question. First off, "legs crossed" means crossed at the knees to me. Not at the ankles. That'd be "ankles crossed" to me. :D
If I cross my legs, I end up with the back of one knee pretty much vertically above, and squashed against, the kneecap of the other knee. If I then "slip one foot up beneath me" then I have to uncross, and then bend one leg at the knee as flexed as it can go (nearly 180 degrees) so that one foot is right under my butt. That's what I think the person in the question is doing. Just my 2c.