第一教 · Dai Ikkyo
Deashi Harai 出足払
Advancing Foot Sweep
Mechanic
Just as uke's advancing foot is about to take weight, tori sweeps it sideways with the sole of his foot while his hands pull down and across. The support disappears at the exact moment it's needed, and uke's body keeps traveling with nothing under it.
Analogy
You stride confidently forward and your front foot lands on a skateboard you didn't see. The instant your weight commits, the board scoots your foot out ahead of you, and the rest of you drops flat where the foot used to be.
Why it maps
The advancing foot is carried away sideways at the precise moment of weight transfer, exactly like the timed sweep of deashi harai.