Ko-soto-gari countered by Uchi-mata.

Counter (Kaeshi-waza).

Techniques
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1. Ko-soto-gari (小外刈), the minor outer reap.
2. Uchi-mata (内股), the inner thigh throw.

About
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Their minor outer reap turns their hips towards you and leaves them briefly on one leg with the other lifted. Uchi-mata attacks that gap before the foot lands.

Step by step (standard right-handed form)
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1. Feel their sole arrive against the outside of your heel as they attempt Ko-soto-gari.
2. Lift the attacked foot clear and keep both grips, staying tall.
3. Step that foot down between theirs and continue turning into them.
4. Pull the sleeve up and across so their chest comes onto your back, hips tight to theirs.
5. Swing your near leg back between their legs and lift the inside of their thigh.
6. They rotate over your hip onto their back; keep the sleeve throughout.

Receiving it
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You attacked at the outside of their heel and they lifted it clear, and now your own reaping leg is committed while their body turns in. What follows is fast: your chest is pulled onto their back, their hips are already under yours, and a leg lifts the inside of your thigh. There is nothing to post on. Stay tight to them rather than reaching for the mat, chin tucked, and slap with your free left arm as your back arrives.

Demonstrations
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Mike Swain: 56:57 Kosoto-gari countered by Uchi-mata https://www.youtube.com/watch?v=_cCAytbmk3g&t=3417s

VI judo notes
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Uchi-mata off a missed reap is close-quarters judo and suits gripped-up work: you are already in contact, so the entry is a turn rather than a journey. The cue is their sole leaving your heel with their weight still committed forward into your hands. Pull the sleeve up and across as you turn so their chest is glued to your back and the lifting leg finds their thigh by feel.

From Waza Book: https://wazabook.com/counters/ko-soto-gari-countered-by-uchi-mata.txt