Ko-uchi-gake (小内掛), the minor inner hook.

Nage-waza (throwing techniques), Ashi-waza (foot and leg techniques).
Required for: 9th Mon (orange belt, British Judo Association); 10th Mon (green belt, British Judo Association); 18th Mon (brown belt, British Judo Association); 1st Kyu (brown belt, JudoScotland).

About
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The hooking form of the minor inner reap: the foot hooks behind uke's heel and stays there while tori drives, often dropping low, rather than sweeping the heel away. Distinct from ko-uchi-gari; gake and gari name different actions.

Step by step (standard right-handed form)
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1. From a right-handed grip, step in so your right foot reaches their right heel from the inside.
2. Hook your foot behind the heel, sole and instep wrapping it, and keep the hook.
3. Drive forward through their centre with your body, hands pressing down and back.
4. They cannot step the hooked foot back, and they fall to their back over it.
5. Keep the sleeve as they land and recover your posture over them.

Key points
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1. Hook the heel and keep it hooked
2. Drive through their centre, low and direct
3. Common in competition as a dropping attack; drill the drive standing first

Receiving it
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A foot wraps your heel and stays while their body drives in: the trapped-foot feeling tells you a gake from a gari. Sit into the backward fall, chin tucked, and slap with your free arm.

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