It's from the good folks at Abhaya Martial Arts in Canada.
The key step in the sweep, the part the Abaya purple belt points out as typically problematic is escaping the hips to the outside. The point that Rodrigo used to emphasize was the scissoring of the legs--bringing the underneath leg back to undercut the far side of the guy's base, and bringing the upper leg over with the same sort of "chuck" that you would use to set up an armbar.
You can see how similar the motion is to the armbar. The armbar-from-the-guard drill we do is also a good warmup for the cradle/pendulum/windmill sweep.
Think of it this way: after you escape your hips, you want to kick your outside leg/foot out, way out, almost behind you. Your upper leg will drive through the guy's armpit as if you were going to reach over with that leg and touch the foot of the upper leg with the foot of the outstretched underneath leg. But, as the Abhaya guy says, you don't want to put the guy on top of your underneath leg. So you scissor it back toward you. The momentum of that "scissoring back" helps drive the upper leg further over, landing you in the mount position.
It's really worth practicing. It is an especially nice sweep for guys who stay low in your guard, or who lean too far forward. That makes it a great compliment to sweeps against "high base" guys like the crossover/kimura combo that Werdum works so well on Lindland.
One thing I remember from when Rodrigo taught us this move is that if you do it right, you will not only end up in mount, but you'll feel like you are almost about to fall on your face. The momentum should be that powerful ...
