第一教 · Dai Ikkyo
Seoi Nage 背負投
Shoulder Throw
Mechanic
Tori turns his back into uke, pulls him tight so uke's weight loads onto his back, then bends and pitches him over the shoulder. Uke's own forward pressure gets redirected up, over, and down in front.
Analogy
A kid rides piggyback, arms wrapped over the carrier's shoulders. Then the carrier suddenly bends sharply forward, and the passenger keeps going, sailing over the shoulders and flipping onto the grass in front.
Why it maps
A load held tight against the back is rotated over the shoulder fulcrum by the sudden forward bend, exactly how uke, pulled onto tori's back, travels over the shoulder in seoi nage.