Tsubame-gaeshi (燕返), the swallow counter.

Nage-waza (throwing techniques), Ashi-waza (foot and leg techniques).
Required for: 5th Dan (black belt, British Judo Association).

About
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The swallow's counter: uke attacks with a foot sweep, and tori's attacked foot dips under the sweep and sweeps back in the same beat, like a swallow turning in flight. The Kodokan treats it as a form of de-ashi-harai.

Step by step (standard right-handed form)
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1. Your partner attacks your right foot with a sideways foot sweep.
2. Curl that foot up and back so their sweep passes beneath it: a small, soft dodge, knee bending.
3. In the same motion, before their sweeping foot lands, sweep it with your sole in the direction it was already travelling.
4. Steer with both hands the same way, carrying their body after their swept foot.
5. They land on their side or back beside you, taken by their own sweep's momentum; hold the sleeve firmly through the fast landing.

Key points
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1. Dodge and counter are ONE motion of the same foot
2. The foot curls under their sweep, then sweeps their sweeping foot
3. Hands steer in the new direction the instant the counter lands

Receiving it
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Your foot sweep meets air, and in the same beat your sweeping foot is itself swept: the counter uses your own momentum, and the fall is as sudden as the throw you attempted. Slap hard with your free arm as your side lands, and trust the held sleeve; this is the fastest landing in the counters.

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

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