Harai-goshi countered by Harai-goshi-gaeshi.

Counter (Kaeshi-waza).

Techniques
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1. Harai-goshi (払腰), the sweeping hip throw.
2. Harai-goshi-gaeshi (払腰返), the sweeping hip counter.

About
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As the sweeping leg comes across, your partner is standing on one leg with their back to you. Hooking that supporting leg from behind and reaping it is the sweeping hip counter, the named answer.
This pairing is a named technique in its own right: Harai-goshi-gaeshi, https://wazabook.com/techniques/harai-goshi-gaeshi/

Step by step (standard right-handed form)
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1. Their back arrives against your chest and their right leg swings backwards along the outside of your right thigh.
2. Bend your knees and take your hips away from theirs, so the sweep has nothing to lift.
3. While their right leg is still travelling, their left leg is carrying everything they have.
4. Hook your left leg round their lower left leg from behind and reap it.
5. They land on their back beside you. Keep the sleeve for the landing.

Receiving it
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You are attacking, and your sweep finds nothing because their hips have gone somewhere you cannot reach. While that leg is still in the air your standing leg is hooked from behind and reaped, so there is nothing under you and you land on your back beside them. Turn into the fall rather than fighting to stay upright on one leg, keep your chin tucked, and slap with the free arm. Sweeping into a partner who has already lowered their weight is what invites this, so feel for their hips before the leg goes.

Demonstrations
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Kodokan: Harai-goshi-gaeshi https://www.youtube.com/watch?v=4U3It-7PPsc
Efficient Judo: Harai-goshi-gaeshi - Demo https://www.youtube.com/watch?v=t2hgLa7BZbE

VI judo notes
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The cue is your own thigh: their sweeping leg brushes it from low to high, and while it does their other leg is the only one on the mat. Keep your leg in contact with theirs and hook that standing leg from behind as soon as you feel the sweep, because searching for it afterwards is what turns the counter into a scramble. Because both of you finish on the mat, agree which way you are working and know where the edge is before you start.

From Waza Book: https://wazabook.com/counters/harai-goshi-countered-by-harai-goshi-gaeshi.txt