There are two quite different ways of trapping the arm, requiring you to fall to the choke/overhook side and the underhook side, respectively. If you fall to the choke/overhook side, you typically have to bait him to expose his top arm to being trapped as you threaten the second hook. This is mostly a way of clearing the arm to work the RNC. I personally fall to the underhook side a lot, adjusting to a more diagonal angle and digging with my heel under his forearm to trap the arm, which is easier to force. From here I usually go to the reverse triangle, or if he bridges into me to stop that, I take the RNC.