Mr Darcy said that there were two motives for Miss Bingley and Elizabeth to choose to walk up and down the room. What were the two motives?

Here is his quote explaining himself,

"I have not the smallest objection to explaining them," said he, as soon as she allowed him to speak. "You either choose this method of passing the evening because you are in each other's confidence, and have secret affairs to discuss, or because you are conscious that your figures appear to the greatest advantage in walking; if the first, I would be completely in your way, and if the second, I can admire you much better as I sit by the fire." Chapter 11 Volume one