Here’s some back mount techniques where you choke your opponent with your legs.
These are demonstrated on the Submission Master Grappling Dummy
Number one starts by feeding your opponents arm down and bringing your leg over it to a triangle choke position (figure 4 legs).
The second one uses the same triangle choke position, but instead you use an s-grip around his head and pull towards you.
The next two submissions are a bit more complex, so you’ll need to watch the video to see how they are performed.