The arm control, commonly known in wrestling as a two on one, is the second most common control position in FSD, after the head and arm control. It's an excellent technique that allows you to control your opponent and apply offensive techniques while providing cover against potential attacks. The arm control is difficult for your opponent to escape from due to the unmatched alignment of your hands (the weak points of your grip are not lined up). While it may look like black shirt in the image above could strike red shirt with his free hand, red shirt can pull or push black shirt in such a way that striking with the free hand is impossible. If your opponent is significantly larger, the arm control still works. Rather than pulling your opponent to you or pushing him away, you simply pull yourself into him or push yourself away.
There are many possible entries to the arm control. My favorite two are the crash and the bong sao from wing chun. A palm blast to your opponent's face will often trigger a cover that lends itself to an easy arm control, and the arm drag can be used to get it from a great variety of situations.