There are a few techniques for slap-and-pop. I can't tell what that tutorial is really going for. The pictures don't align w/ the words, from how I read it.
The pictures look "Flea-style"(Flea didn't invent it, but certainly helped popularize it): Perpendicular thumb, bounces off the string.
There's also "Wooten-style"(what is this, "kung fu"?), in which you keep your thumb parallel to the string and slap through, coming to rest on the string below(what the words say). You can come up through the string too. I think this is a harder style. It's less flashy, but you can get a double-pop at ludicrous speed and is probably easier on your hands. ...maybe.
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