I don't know if this is the proper term, but this is what I'm going to call it because that's exactly what it is! In the .gif below you see the master of this technique in Randy Couture. It's basically a momentum move in which a smaller guy uses his body to get his opponent off balance and to the ground. You'll notice that Couture circles behind Gabriel Gonzaga at UFC 74, but he doesn't stop there. He keeps his momentum going so that Gonzaga's center of gravity is thrown off, and then uses his left leg to trip Gonzaga to the mat in back control. I was tried to find a .gif of Forrest Griffin taking down Shogun Rua at UFC 76, but I wasn't able to do so. He used a similar technique, except he had double under-hooks and got Shogun's momentum going backwards. Then just basically used one of his legs to trip Shogun down to the mat. It's an excellent technique when guys have good take down defense.
Another variation of this move is after the jump.
Here we have Dan Henderson and Ricardo Arona, and Hendo has Arona with his hands clasped. As he goes to take Arona down, he just posts his right leg out just a little bit which is enough to throw Arona's balance off for the take down. I guess it's not coincidence that both Couture and Henderson are Greco Roman guys :). More fighters should be using these types of techniques because everyone's sprawl is getting better, and other fighters are learning to throw that knee ala Aldo/Swanson.
I really wished I could have found the Griffin/Rua .gif as that was the best case scenario of what I was trying to get across.
.gifs courtesy of MMACore