Closed guard, bottom
Kimura

A shoulder lock that shows up everywhere. From closed guard it's the punishment for posting a hand on the mat: you trap the wrist, figure-four the arm and turn it behind their back.
Step by step
Catch the posted hand
Break their posture so they put a hand on the mat. Grab that wrist with your same-side hand, four fingers on top.
Sit up and reach over
Open your guard and sit up on your elbow. Reach your free arm over their shoulder, past the triceps.
Lock the figure four
Grab your own wrist from underneath. Both your thumbs point the same way.
Fall back at an angle
Fall back to the mat while shifting your hips out to the side so you're nearly perpendicular to them. Throw your leg over their back to stop them rolling out.
Turn it slowly
Keep their elbow tight to your chest and rotate their hand up behind their back, toward their head. Go slow: the tap comes fast.