That's pretty sick and I'm not about to go on a hate on Werdrum tangent here, the guy is one of the best grapplers in the world, but that armbar was a little sloppy.
He had his feet crossed and he wasn't using his legs to pull Magalhaes arm toward him and it looked like he was a little deep in on the arm, not to mention he didn't pinch his knees all that much.
The rash guard looked like it played into that a lot though, Werdum couldn't get his arm positioned properly so that his hips could flex to break it, which is why you should use your legs to pull the arm out some, you don't need to lift your hips nearly as far for the same pressure. Oh and it didn't look like Werdum ever actually tried to break it.
Bolded part it's prabably right.
Werdum knew he was winning, so he probably opted for saving Vinny's arm.
Lyoto Machida, former UFC LHW hamp
Future UFC MW Champ
Benavidez, Barao, Aldo, Nate, Nick, Lyoto, TBD, JDS
Forever hanging in Anderson Silva nuts