The under hook is used constantly in the higher levels of wrestling, but still under-utilized with the young ones. It's very easy to go for, and gets you closer to your opponent than a traditional collar tie. It also gives you position over your opponent, or in this case, under.
Your arm will go under your opponent's arm and then wrap above his shoulder, creating a good amount of control. This especially works well when you get an angle and move him around with your free hand either placed on the head or with a far wrist controlled.
Your left arm will hook underneath his right arm. Your hand should grab the shoulder tight and your elbow should be up in the air to gain more control of the situation. You should have head position, otherwise your move selections are limited.
Your opponent will have an Under-Hook, so you should have an Over-Hook. Chop the elbow down as much as you can, use both arms if you have to. You must get control of this situation or go somewhere else.
Using your Over-Hook, you will chop the arm down while pulling the far arm in with your left hand. With the Over-Hook arm, grab the far arm as high as possible and pull them together as tight as you can. This will trap both arms with your right arm, allowing your left hand freedom to attack.