Hara-gatame (腹固), the stomach armlock.

Kansetsu-waza (joint locks).
Required for: 2nd Dan (black belt, British Judo Association); 3rd Dan (black belt, British Judo Association); 4th Dan (black belt, British Judo Association); 5th Dan (black belt, British Judo Association).

About
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The armlock levered over the stomach: uke's straightened arm is held across tori's abdomen and the lock comes from tori turning their body, not from pulling with the hands. Reachable standing as well as on the ground, which makes it one to treat with particular care. Competition joint locks are restricted to the elbow and to senior age groups; learn and practise this only under instruction.

Step by step (standard right-handed form)
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1. Take your partner's right wrist in both hands with their palm turned upward.
2. Draw the arm across the front of your body so their elbow rests against your stomach.
3. Straighten the arm gently and keep the wrist high, so the joint sits square on your abdomen.
4. Turn your hips slowly away from them; your body, not your grip, is what applies the pressure.
5. Stop the moment they tap, and step back to give the arm room rather than simply letting go of the wrist.

Key points
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1. Their elbow sits ON your stomach, not on your ribs
2. Hold the wrist with both hands, thumb up
3. Turn your body to apply; the hands only hold

Receiving it
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Your arm is drawn across their body and your elbow finds a hard surface, then their whole body turns. The lock arrives fast because their hips are stronger than your arm. Tap the instant you feel the elbow load, on their body or the mat, or call out; do not try to spin out of it, which loads the shoulder as well as the elbow.

Reference pages elsewhere
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Hara-gatame on the IJF technique reference (International Judo Federation): https://judo.ijf.org/techniques/Hara-gatame

From Waza Book: https://wazabook.com/techniques/hara-gatame/