This position is like the side mount and you can easily move into this position if you're in the side mount and your opponent tries to bring their legs underneath you.
Numerous submissions can be applied from this position.
With your legs out straight you should force your chest onto your opponent, this will make it hard for them to breath. Hold onto your opponents belt to make it hard for them to turn underneath you, try to keep their hips pinned to the ground.
With this variation simply bring your knees up. Keep your knees apart to maintain a good base so your opponent can't roll you off.
A close-up of the grip on your opponents belt.