Kawazu-gake (河津掛), the one-leg entanglement.

Nage-waza (throwing techniques), Yoko-sutemi-waza (side sacrifice techniques).
Required for: 5th Dan (black belt, British Judo Association).
Also known as: grapevine takedown (english).

About
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A prohibited throw in which tori winds a leg around uke's leg from the inside and falls backwards, taking uke down with the leg still entwined. Documented for recognition only: the entangled fall loads uke's knee unpredictably, which is why the technique is banned.

Key points
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1. Recognise it: a leg wound around yours as the attacker falls backwards
2. Banned in competition; attempting it is direct hansoku-make
3. Never complete an entangled backward fall in practice; release and step out

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

From Waza Book: https://wazabook.com/techniques/kawazu-gake/