Ashi-gatame (足固), the leg armlock.

Kansetsu-waza (joint locks).
Required for: 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 applied with a leg: tori fixes uke's wrist and lays a leg across the elbow, so the lock is applied by lowering the leg rather than by pulling. Strong, because a leg is stronger than an arm, and for that reason worth applying with unusual 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. From beside your partner on the ground, take their near wrist in both hands and turn the palm upward.
2. Keep that arm straight and draw it clear of their body so the elbow is exposed.
3. Lay your near leg across the elbow, the back of your knee or your shin resting on the joint.
4. Hold the wrist up and lower the leg slowly, so the joint is pressed against its hinge.
5. Release the moment they tap: a leg applies far more than a hand and gives less warning.

Key points
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1. Wrist fixed first, palm turned upward
2. The leg goes ACROSS the elbow, not the shoulder
3. Lower the leg slowly; it has far more power than a hand

Receiving it
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Your arm is straightened and then something much heavier than a hand settles on the elbow. There is almost no margin here. Tap at the first pressure, loudly, with your free hand or your feet, and do not wait to see whether you can turn out of it.

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

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