Marlon Sandro beat Matt Jaggers at Sengoku 7th Battle with a standing arm triangle (known in the judo world as kata gatame).
Most of the match was spent on the ground with Sandro dominating position but not scoring much damage and not going for many submissions.
One theme that reoccurred throughout the fight was reversals of position with Jaggers escaping from bottom position several times and Sandro just as frequently reversing again to regain top position.
Jaggers' undoing came immediately after he scored another reversal. Sandro had gained top position and was working for an arm triangle side choke. Announcer Bas Rutten was openly heckling Sandro throughout the fight and had dismissed Sandro's chances of scoring a submission with a side choke on Jaggers.
Just then Jaggers rolled and threw Sandro off, as they stood, Sandro immediately went for the side choke again and this time he nailed it.
More on how Sandro executed the choke and how Jaggers exposed himself to danger in the full entry.
Here's how wikipedia explains the arm triangle:
Arm triangle choke (or simply arm triangle) is a generic term describing blood chokeholds in which the opponent is strangled in between his or her own shoulder and the performers arm. This is as opposed to the regular triangle choke, which denotes a chokehold using the legs, albeit with a similar mechanism of strangulation against the opponent's own shoulder. An arm triangle choke where the performer is on the side of the opponent and presses a forearm into opposite side of the neck of the opponent is called a side choke, such as from the kata-gatame hold.
Gif by Chris Nelson from the MBC ESPN broadcast of Sengoku 7, March 20, 2009.
Here you can see Sandro finishing the choke. He's got his left arm wrapped around Jaggers' neck and his right arm trapping Jaggers' left. He's got his arms in a figure four with his left hand hooked in the crook of his right arm. This gives him the leverage to squeeze the choke.
As BE reader Rundownloser points out in the comments, Sandro throws his legs around Jaggers' hips for even more control, pulling guard and finishing the choke.
The choke relies on Jaggers' left arm and shoulder cutting off blood flow in one artery while Sandro's left arm cuts off the other. Squeeze for a minute and viola! instant unconsciousness as the oxygen supply is cut off to the brain.
Jaggers got into trouble when he extended his arms to post on the ground and stand up. In jiu jitsu theory they teach to keep the elbows in close to the abdomen. Its all about leverage and power -- the more extended your arms are, the less power you have and the more you're exposed.
But its a double edged sword, in all of Jaggers' escapes from bottom position throughout the fight he had successfully used an extended arm as a lever to help him flip Sandro over. In fact, Sandro had previously isolated Jaggers' left arm and had been working for the side choke on the ground when Jaggers used his left hand to push on Sandro's shoulder, buck his hips and roll Sandro off.
For a split second the hold is broken and Jaggers is free. The he reaches down and posts up with his hands so he can stand. One thing I'm learning about jiu jitsu is that posting your hand on the ground is alot like grounding yourself when you're touching a high-tension electric wire -- not a smart thing to do!
Jaggers' extension of his arms allows Sandro to sink the hold in deep and that's the key to the power of the choke. Once Sandro has his shoulder buried in Jaggers' armpit, that left arm is helpless and utterly trapped. From there its just a matter of cinching his own left arm tighter and tighter around the throat, locking in the figure four and squeezing.
As always, I'm learning about this as I write so please do correct me where I've gone astray.