第一教 · Dai Ikkyo
Osoto Gari 大外刈
Major Outer Reap
Mechanic
Tori rocks uke's weight back onto one heel, then swings his own leg through like a scythe and reaps that supporting leg out from behind. With the weight already behind the base and the base suddenly gone, uke falls straight back.
Analogy
You plant one boot on a sled parked at the top of a hill and lean back to shout at your friends. The sled sets off downhill behind you and takes that boot with it. Your foot goes one way, backward and up, and the rest of you goes down flat in the snow.
Why it maps
Your weight is already committed backward over one foot, and then that foot is carried away backward under you, which is exactly the direction osoto gari reaps the supporting leg.