Yoko-wakare (横分), the side separation.

Nage-waza (throwing techniques), Yoko-sutemi-waza (side sacrifice techniques), Gokyo 5th set (Dai-gokyo).
Required for: 1st Dan (black belt, British Judo Association); 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).

About
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Tori drops onto the side across uke's front, both legs extended, and pulls uke straight over the body.

Step by step (standard right-handed form)
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1. From a right-handed grip, draw your partner's balance forward onto their toes.
2. Drop your body across the front of their feet, falling onto your right side with both legs extended together past the outside of their right side.
3. Your body lies like a bar across their line of travel; as you drop, pull both hands hard over your own body towards the mat beyond you.
4. Their weight, already forward, is pulled straight over your lying body.
5. They roll over you and land on their back on the far side; keep pulling through the roll and hold the sleeve as they land.

Key points
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1. Drop across uke's front with both legs extended
2. Pull uke over your body with a sharp two-handed action
3. Fall onto your side, never flat on your back

Receiving it
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Your partner drops across the line of your feet, body lying sideways in front of you, and pulls you straight over them: your own forward balance does the rest. Fly with it rather than stalling on top of them, tuck your chin, and take the roll over their body onto your back on the far side, free arm slapping.

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

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