Here are a few options to get from an arm drag to a choke. There are Brazilian Jujitsu techniques, BTW.

This is a straight forward how-to video that shows you a knee bar sweeping technique from an open stacked guard. Depending on how tight and fast your are, this can be a sweep or fall into an actual knee bar.

In this video, Mark Plavcan teaches you a half guard pass when the bottom guy gets the underhook.  Simple, effective technique for MMA, BJJ, jujitsu or any submission grappling style.