Here’s some back mount techniques where you choke your opponent with your legs.
These are demonstrated on the Submission Master Grappling Dummy
Note: Most experienced submission wrestlers won’t wrap their arm around their opponents neck inside the guard against another submission wrestler because their opponent will know how to easily counter it.
However, it is still very likely to happen on the street against a non-submission fighter or a wrestler.
First, if your opponent tries to choke, you can drive away with your legs so he can’t get good leverage for a choke.
Next, you can reverse / sweep your opponent with a single leg elevator (butterfly sweep).
Also, you can do the same sweep with a double leg butterfly sweep.
If you go for the above technique and your opponent’s body weight is too far back for the sweep, drive him up with your forearm to his throat, sit up, bump your hips in, then fall back again for the sweep.
You can also easily do a straight arm elbow lock from this guard position, as well.