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.

No word is omitted. Note: I've picked images that are acceptably modest by US standards. By the way, can you see these or are you blocked? "Legs crossed" has two different meanings. "Relaxed back with her legs crossed" probably means sitting in a chair with the legs crossed at the ankles, like this: http://aladylikeyou.com/wp-content/uploads/crossingyourlegs.jpg In this second image, we could say that she is sitting "with both legs under her," which is also called "legs crossed." Sometimes this is called "sitting Indian style." http://st.depositphotos.com/1712366/3508/i/950/depositphotos_35089957-stock-photo-woman-sit-cross-legged-tablet.jpg In your sentence, I think woman is sitting on a chair in the first position, and then has crossed one but not the other leg under her--halfway into the second position. I can't find a good picture.