One of the most important elements in order to become a successful finisher from the Back position is knowing how to clear your opponents defenses, a.k.a. hand fighting.
Trapping one of his arms with your leg is always a great option, but you may still need to deal with his grips in order to finish the choke.
In the video above I explain some strategies that you should be using if you wanna have a high finishing rate from the Back position.