This is the starting position. My knee is just over his hip bone and my hands are "checking" his arms to prevent either the underhook or the cross face. I have to thank Ryan Gregg for the detail of the left hand checking the opponent's right arm to prevent the underhook.

If he tries to back out of the position, I can usually get in deep and get the underhook (that foot trap in the next picture is tighter than it looks), but if he forces his way back, I switch to an open guard (usually shins on the biceps).

When you feel that you have the opportunity to enter, go for your underhook and drive your left shoulder as close to his right hip as possible.

I underhook and grab the belt, but I am working on changing that grip to my opponent's left trap instead (as per advice Shawn Willians gave me). Grabbing the far trap feels a bit awkward for me, but I need to add it because it prevents him from overhooking your arm and getting the choke.

Now pull your knees forward as you scoot down as much toward his back as you can. You want to be as small as possible so that his hips are over you.

I now initiate the attack and keep the opponent off balance by rocking him. The rocking means I am always trying to get his body higher and higher over my upper body. I want his hips over my chest, if possible, so he feels light and I have an easy path to my knees or his back. I also go for that sweep to my left, and if he bases to prevent it, I rock the other way and the momentum helps me get to my knees.

Once you are as small as possible under him, switch your legs to this position.

Putting my left foot inside his right leg allows me to switch to the sweep to my left easily. To sweep to my left I will use my right leg as the base that drives me, while my left leg blocks his leg from escaping. I also find it easier to get to my knees when my right leg is free to shift out.

Then try to get to his back by getting on your elbow.

Getting on my right elbow keeps him from being able to flatten me out. If he really makes an effort to do so, it sets up the sweep to the left so easily.

Or block his far leg with your right hand, get to your knees.

I try to pull their right leg under me with my left leg (that is hooked). Sometimes they get away and it is more of a tackle, and other times I am able to slide their right leg underneath me with the left leg and it is an even tighter and more effective sweep.