2 variations of the armbar

Everybody knows the basic armbar where you hold the opponent's
arm to your chest. What about the underarm variation where
you're holding the opponent's arm under your own arm? Is there a
reason why I don't see that as often? Does it leave openings for
your opponent?

*edited - i'm asking because it seems easier to secure the arm,
plus it also seems to take less effort to sink in the submission.

yeah i'm referring to holding it underarm, wrapping your arm
around his arm and using your other hand to grip it as well, it's a
pretty solid grip.

With the first variation, you have much more leverage. It's that simple.

Oh, sorry, I misread that first post. I thought you meant, the "elbow press" armbar that you do from an underhook, i.e. this one:

http://www.judoinfo.com/images/kansetsu/hiza_gatame.gif

My 2 cents:It's a great armbar when you have it right. But, I think you don't see it as much because it's not there from as many positions and setups as the regular armbar. It usually takes an extra step to trap tha arm and is a little slower setup.