Definitely a technique that's difficult to explain, let alone explaining over the internet and not in person.

As Lindholm said, you clip the string with your thumb after you pick. Well, its something moreso done simultaneously. You hold the pick in such a way to where the pick and your thumb both clip the string. And not only that, you get different results based on where you hit string, just to make things more confusing.

Honestly, I ended up understanding how to do it when I did it on accident :p