O-uchi-gari (大内刈), the major inner reap.

Nage-waza (throwing techniques), Ashi-waza (foot and leg techniques), Gokyo 1st set (Dai-ikkyo).
Required for: 6th Mon (yellow belt, British Judo Association); 7th Mon (orange belt, British Judo Association); 5th Kyu (yellow belt, JudoScotland); 4th Kyu (orange belt, JudoScotland); 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: inside trip (wrestling).

About
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Tori reaps uke's leg from the inside with a circular action of the calf, driving uke to the rear as the supported leg is taken away.

Step by step (standard right-handed form)
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1. From a right-handed grip, step your left foot forward and slightly across, so your body comes close to your partner's front, then bring your right foot between their feet.
2. Curl your right calf around the inside of their left leg, at the knee or just below, and reap it out to your right in a circular motion, in the direction their toes point.
3. As the leg is reaped, drive with both hands down and back towards their left rear corner, your chest following.
4. Keep your supporting left knee soft and your weight forward so you stay balanced over them.
5. They fall onto their back towards their left rear corner. Slide your grip up the sleeve and support it as they land.

Key points
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1. Reap the leg from inside in a circular, hooking arc
2. Drive uke backwards over the trapped leg with both hands
3. Keep your own supporting knee soft for balance

Receiving it
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A leg curls inside your left thigh and hooks it out while your weight is pushed back: you fall backwards towards your left rear corner. Tuck your chin, sit into the fall rather than reaching back for the mat, and slap with your free arm. Never post your hand behind you straight-armed; that is how wrists and elbows get hurt.

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

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