I'm fascinated by this theory (put forth in Kid Nate's great, provocative article http://www.bloodyelbow.com/2010/8/8/1611866/opinion-did-chael-sonnen-quit) that some fighters give a single, sly tap when a sub is being applied hoping that the ambiguous tap will either cause their opponent to loosen the hold, or to allow plausible deniability if the ref stops the fight. 

Some people think that that's what Sonnen was trying to do against Anderson, but that Josh Rosenthal saw the "quick tap," stopped the fight, and wouldn't buy Chael saying he didn't tap.* I think he came very close to buying it, though.

Sonnen's teammate and cornerman Matt Lindland once successfully pulled this off against Murilo Bustamante: Lindland tapped, referee John McCarthy stopped the fight, then Lindland denied tapping.

(gif from Bullock's thread on MMAnews.com)

Apparently Big John restarted them from a standing position after this!

Anyway, here's the gif (stolen from dancingChicken in another thread) of Maia's triangle on Sonnen. Watch Sonnen's right hand when it comes up onto Maia's chest. If I wasn't looking for it, I might not have thought anything about it, but to me, it looks like a quick little pulse that the ref and Maia ignore. It could definitely be that he's just feeling for Maia or leaning on his hand, but that's not what it looks like to me. 

* I think Rosenthal wasn't sufficiently decisive in stopping the fight. When he came in and touched the fighters, he didn't wave his arm like he usually does at the end of a fight to call for the bell. Instead, he immediately started talking to Chael. I thought for a moment that Rosenthal was going to restart the fight, but that Anderson walking away prevented that. As Anderson is standing up, you can hear Rosenthal yelling "Stop!" repeatedly, as if he wanted Anderson to stay on the mat. The only reason he would want that would be if he was considering continuing the fight.