Dan Faggella is a BJJ Academy Owner, Brown Belt Pan Am Champion, and Respected Expert on Beating Bigger Opponents as a Lighter Grappler. In Addition to Writing for Jiu Jitsu Magazine, Jiu Jitsu Style, and Other Publications, Dan’s Free DVD “"The Bigger They Are, The Harder They Fall” can Be Found at www.MicroBJJ.com 
"I learned how to do an under hook properly after my Brock Lesner match." – Frank Mir
How to PROPERLY do an Underhook from Bottom
Frank Mir admits that he did not know how to properly get into an EFFECTIVE underhook position until after Brock Lesner made his face look like raw steak. In fact MOST of us don’t know the proper position for an EFFECTIVE underhook technique.
Sure, we all know what it looks like and it’s basically just getting your arm under your opponent’s armpit right? WRONG!
UNDERHOOK with your HIPS
Today in this video former UFC Heavyweight champion and BJJ Giant Frank Mir breaks down the proper way to do an effective under hook. I stress the word effective because a lot of has have been in position (especially against a superior wrestler or larger opponent) where we get our arm “underhooked”, but then….nothing happens? Why isn’t this working? Usually we struggle, exhaust energy we don’t have to, and then either MUSCLE our way out, or not.
Frank here stresses one KEY MISTAKE that caused him to be in big trouble in his second match with Brock Lesner….and that is that he kept his legs CROSSED and locked in half guard position, not allowing him mobility to escape and make an EFFECTIVE underhook.
Frank Mir Underhooks for MMA, Tip #1:
After you get your underhook position with your arm, open your guard! Without this kind of mobility you're functionally doomed, and you don't be able to escape your hips the way you should.
Frank Mir Underhooks for MMA, Tip #2:
GET DEEP! What Frank stresses here is that not only is he opening his guard when he lands that under hook, BUT he hip escapes very low, almost taking his opponent back. This allows him to ESCAPE and gain LEVERAGE. The opponent cannot flatten you from here, as he could if you did not do the above.
I've written about this before in a blog post about quick ways to escaping side control against bigger and stronger opponents  - and it's a nuance that gets underplayed a lot. I always say to reach with each underhook like you're trying to reach for the opponent's belly button.
After Frank nails underhook secrets #1 , he goes into his options. Options in his attacks, but that depends on way he ANCHORS his hand for the underhook. If he wants to stand, he anchors it high on the trap. If he wants to go for a double leg or take the back, he anchors it low, etc.
GREAT over looked detail Frank shares with such a common move. I’m confident that if I have to fight Brock Lesner…I will know what to do! Thanks Frank!
PS: Ricky Lundell is one of Mir's trainers and in my opinion he's one of the best grappling coaches in the game today.