Trick is VERY easy to perform. There are a few different ways (especially on video), but the best way to do it is as follows:
As with most tricks, the delivery is the most important aspect to making it believable.
1. Shoe is tied but the bow is tucked behind your pant leg.
2. Another shoelace is tied in a bow around one of the laces, tongue or wherever looks right. and placed in front of the pant leg so as to appear that it is the one attached to the shoe. This shoe lace is attached to string (or floss) inside the pant leg.
3. Floss can be routed into your pocket (note that the guy in the video has his shirt tucked on his right side so he has access to the pocket.
4. The important part of the trick: Untie the dummy shoelace and tell people that they have to watch really close, because it is going to happen really quickly. Flop around the untied shoelace for awhile, keep telling it's going to happen quickly. When all the focus is on your foot, you need to simultaneously pull the floss up (so the dummy shoelace goes up your pant leg) and move your leg enough that the actual shoelace comes in front of your pant leg.
Very simple trick - many ways to do it, but that is how I learned.