She knew that Scott was wondering how she was doing this, but this was really not the time or place for her to try and tell him anything about her or her powers.
"I believe I can keep him contained for awhile. I don't think it's something we could take back to the Institute." She was not about to tell him that she could keep it contained as long as they needed by using the Phoenix Force. If she wasn't using it, she'd pass out from the strain on her powers in about fifteen minutes.