What is the test for federal courts to review moot cases?

The federal courts' "impotence 'to review moot cases derives from the requirement of Article III of the Constitution under which the exercise of judicial power depends upon the existence of a [live] case or controversy.' " Id. (quoting Liner v. Jafco, Inc., 375 U.S. 301, 306 n. 3, 84 S.Ct. 391, 394 n. 3, 11 L.Ed.2d 347 (1964)). When the court ceases to be able to affect the legal relationship between the parties upon the complaint before it, that is the end of the inquiry. The court stands like an unplugged appliance. The judicial power is lost, and there is no jurisdiction for a consideration of the equities.