User blog comment:IamJakuhoRaikoben/Chapter 379: Tartarus Arc, Part 3: Underworld King, Review./@comment-5233713-20140404171755/@comment-7418318-20140404173112

I guess it's possible that her desperation gave her the power to force the gate close through Franmalth' interference, but I hope the contract version is the true one.

It would show that Lucy was even willing to give up being with them if it meant keeping them save.