Yoko-gake (横掛), the side hook.

Nage-waza (throwing techniques), Yoko-sutemi-waza (side sacrifice techniques), Gokyo 5th set (Dai-gokyo).
Required for: 17th Mon (brown belt, British Judo Association); 1st Kyu (brown belt, JudoScotland); 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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Tori sweeps uke's ankle sideways while falling to the side, so both go down together with uke thrown flat.

Step by step (standard right-handed form)
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1. From a right-handed grip, draw your partner's weight onto their right foot, their balance leaning slightly to that side.
2. Sweep the outside of their right ankle with the sole of your left foot, sweeping across in front of your body.
3. As you sweep, let yourself fall onto your left side, your fall and the sweep happening together: your dropping body powers the sweep.
4. Pull the sleeve down towards your own falling shoulder so their body tips flat.
5. Their feet are taken sideways and they land flat on their back beside you while you lie on your side, still holding the sleeve.

Key points
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1. Sweep the ankle with the sole as you begin to fall
2. Pull uke's sleeve down towards your own falling side
3. Sacrifice fully: your fall is what powers the sweep

Receiving it
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Your ankle is swept sideways at the same instant as your partner's body drops: their falling weight powers the sweep, and you go from standing to flat with very little in between. The landing is flat on your back beside them. Slap hard with your free arm exactly as your back lands, breathe out, and keep your head off the mat with your chin tucked.

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

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