Yayo pretty much explained it. Fishing line hooked up to the ends of the untied laces, while you shake your leg, you raise your arms to do the "look ma, no hands!" gag which ends up pulling the loose ends up... shaking causes your pant legs to rise slightly to reveal the tied up laces hiding under.
Now you got me crying!
Damn, the know must be predone there in some way! ^g^
http://www.murphysmagicsupplies.com/video/clips/ANIMATEDSHOELACES-VIDEO1.wmv