Sumi-otoshi (隅落), the corner drop.

Nage-waza (throwing techniques), Te-waza (hand techniques), Gokyo 5th set (Dai-gokyo).
Required for: 5th Dan (black belt, British Judo Association).

About
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A hand throw to uke's rear corner: as uke's weight settles onto the heels, tori steps in and drops uke straight down into the open corner.

Step by step (standard right-handed form)
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1. From a right-handed grip, move so your partner's weight settles back towards their right heel, their balance leaning to their right rear corner.
2. Step your left foot forward and outside their right foot, your body coming close past their right side.
3. Drive both hands down towards their right rear corner: sleeve hand pressing down and back, lapel hand pushing through their chest line towards the same spot on the mat.
4. Nothing touches their legs; your step and the two-handed drive alone send their weight past their heel.
5. They sit back into the corner and land on their back; hold the sleeve up as they land so the fall is broken.

Key points
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1. Break balance to the rear corner over uke's heel
2. Step deep behind uke's line as you drive down
3. Throw with both hands together: there is no leg contact

Receiving it
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No leg touches you: a step past your side and a two-handed drive send your weight past your right heel into the rear corner. It feels abrupt, like a chair being pulled away. Sit into the fall, chin tucked, back rounded, and slap with your free arm; do not reach a straight arm behind you to save it.

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

From Waza Book: https://wazabook.com/techniques/sumi-otoshi/