Well, this is a very basic explanation of how I try to emulate it.
When doing this, remember that your thumb should be moving parallel to the body instead of perpendicular like when you are normally slapping (ala Flea).
You want to strike the string but keep going through it to the next string and use that next string as a stopping point (with the 1st string, you will just have to stop yourself). Then, after stopping rake back up into the string you had just struck. I like to use the side of my thumb joint on the way down and then the nail on the way back up. Hope this helps.