QUESTION: Random Mendes guard pass

In the video link, at 1:25, Mendes goes into a rolling guard pass that is really sweet. I have analyzed this short 3 second clip over and over and managed to hit it in rolling a few times. However, I don't know if I am doing it right and/or like Mendes does in the video. I just go for it and it works out. Anyone care to shed any light on this?

http://www.youtube.com/watch?v=D1Xo9HTqS7g

Caio Terra has a video on this from his half guard compilation. He shows it as a counter to the hook sweep from butterfly. It's cool, but you have to be careful not to just get swept. I'm still playing with the details myself, but the key is to have your trailing leg catch and drive forward your opponent's sweeping leg.