This term came from the movie "Back to the Future Part II." In the movie, when Marty puts on his Nike MAG shoes, the power laces work their magic and make the shoe fit his foot tightly. This is the idea with these jeans when girls (or some guys) put them on. At first, when putting on these pants, they are loose but like with the shoes they immediately tighten on command and fit tightly on you. It is not sure if jeans like this actually exist but it is where the term originated.
"I know! Her ass looks so good in them!"