Sankaku-jime (三角絞), the triangle strangle.

Shime-waza (strangles).
Required for: 18th Mon (brown belt, British Judo Association); 1st Kyu (brown belt, JudoScotland); 1st Dan (black belt, British Judo Association); 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).
Also known as: triangle choke (bjj) - One of BJJ's signature submissions, applied from guard and many other positions; the figure is the same..

About
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The triangle strangle: uke's neck and one arm are enclosed in a triangle of tori's legs, one leg's knee-pit locking over the other's instep, the trapped shoulder completing the strangle. Strangles are restricted by age in most federations; learn and practise this only under instruction and supervision.

Step by step (standard right-handed form)
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1. You are on your back with your partner between your legs, one of their arms in and one arm out: trap their right arm inside as their head comes down.
2. Swing your left leg over their right shoulder so the back of your knee crosses the back of their neck.
3. Fold that leg and hook its shin behind the knee-pit of your right leg, the right leg folding down: the triangle is formed around their neck and trapped arm.
4. Square your hips towards them and draw their trapped arm across their own neck, or their head gently down.
5. Close gradually by squeezing the legs and drawing with the hands. Open the triangle completely the moment they tap, and let them up.

Key points
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1. The triangle encloses the neck AND one arm, never the neck alone
2. Lock the knee-pit over the other instep and square the hips
3. Close gradually with the legs and a draw of the head or arm

Receiving it
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A leg crosses behind your neck, locks into the other, and your own trapped shoulder becomes part of the strangle: the pressure comes from the legs and builds smoothly. Posture up and free the trapped arm early if you can. Tap immediately and clearly the moment it is properly on; never hold out to prove toughness. Tell your instructor if you ever feel light-headed afterwards.

VI judo notes
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Built from full contact on your back, so the triangle can be formed entirely by feel: the back of your knee across their neck, then your shin hooked behind your own knee-pit, a lock your legs can confirm without any help. Square your hips towards them and close slowly. Open the triangle completely at the tap so they can get their head clear.

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

From Waza Book: https://wazabook.com/techniques/sankaku-jime/