De-ashi-harai (出足払), the advancing foot sweep.

Nage-waza (throwing techniques), Ashi-waza (foot and leg techniques), Gokyo 1st set (Dai-ikkyo).
Required for: 1st Kai (white belt with a red stripe, JudoScotland); 2nd Kai (white belt with a yellow stripe, JudoScotland); 3rd Kai (white belt with an orange stripe, JudoScotland); 8th Kai (white belt with a black stripe, JudoScotland); 9th Kai (red belt with a white stripe, JudoScotland); 2nd Mon (red belt, British Judo Association); 6th Kyu (red belt, JudoScotland); 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
-----
A pure timing throw: uke's advancing foot is swept sideways at the instant it touches the mat, before it can take weight.

Step by step (standard right-handed form)
-----------------------------------------
1. Take a standard right-handed grip: your left hand on your partner's right sleeve at the elbow, your right hand on their left lapel.
2. Move with your partner and feel for the moment their right foot swings forward to step: the throw lives in the instant just before that foot takes their weight.
3. As their right foot is about to land, sweep it with the sole of your left foot, catching the outside of their ankle and carrying the foot across in front of their other leg, in the same direction they are travelling.
4. At the same time pull both hands in the direction of their travel, sleeve hand drawing down and across, so their upper body keeps moving while the foot is taken away.
5. Their legs swing from under them and they land on their side or back beside you. Keep hold of the sleeve so the landing is supported.

Key points
----------
1. Sweep as the foot lands, not after it has taken weight
2. Use the sole of your foot flat against uke's ankle
3. Pull with both hands in the direction uke is already moving

Receiving it
------------
There is almost no warning: as your foot swings forward to step, it is swept across before it can take your weight, and both legs go together. The fall is sideways and fast. Your right sleeve stays held, so slap with your free left arm as your side meets the mat, and let the held sleeve carry some of your weight.

VI judo notes
-------------
A sighted judoka times this by watching the step; you time it through the sleeve. With grips already taken, the rhythm of their feet travels up the arm, and the moment to sweep is the small lightening in the sleeve as the foot swings forward. Drill it moving up and down the mat with a partner until the tempo, rather than the sight of the foot, is the cue you act on.

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

From Waza Book: https://wazabook.com/techniques/de-ashi-harai/