O-uchi-gaeshi (大内返), the major inner reversal.

Nage-waza (throwing techniques), Ashi-waza (foot and leg techniques).
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 counter to O-uchi-gari: as uke reaps deep inside, tori steps the attacked leg back out of the hook, brings their own leg round behind uke's standing leg and drives them over it. Classified by the Kodokan as a foot technique, because the leg does the work.

Step by step (standard right-handed form)
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1. Your partner attacks O-uchi-gari, hooking their right leg deep inside your left.
2. Step your left leg backwards out of the hook, keeping your hips square and your weight on it.
3. Their attacking leg is committed and their standing leg is loaded; bring your right leg round behind it.
4. Reap that standing leg forward while both hands drive their chest backwards.
5. They fall on their back where they stood; keep the sleeve and lower them under control.

Key points
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1. Step out of the hook before it takes your weight
2. Their standing leg is now the target
3. Drive their chest back as your leg goes in

Receiving it
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Your reap finds nothing, and before you can put the leg down your standing leg is taken from behind. The fall is straight backwards with both legs gone, so it is a full back breakfall: chin tucked, arms ready, and let the held sleeve turn you slightly onto one side if it can.

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

From Waza Book: https://wazabook.com/techniques/o-uchi-gaeshi/