When they are imprisoned in the dungeon, Fiona finds out about Charming's plans for Shrek. She realizes that it is essential for them to break out and tries to rally the other girls to her cause. Snow White takes charge and says "Ladies, assume the position!" What are they doing?
Answer: They are waiting to be rescued.
The princesses all assume the positions they were in when rescued by their princes. Snow White lies down the same way that she did in her glass coffin, just before being kissed by her prince, and Sleeping Beauty goes to sleep. This is a funny comment on the traditional role of princesses, and women in general, in fairy tales. Fiona, however, has a much less passive role in mind.