第三教 · Dai Sankyo
Kosoto Gake 小外掛
Minor Outer Hook
Mechanic
Tori hooks a foot behind uke's heel from the outside and pins it to the mat, then drives their weight straight into uke's chest. With the foot anchored and unable to step back, uke topples backward over their own trapped leg.
Analogy
One boot is sunk to the ankle in deep mud when a friend gives you a playful shove on the chest. The rescue step backward never comes, the boot won't budge, so you tip straight over it like a fence post, flat onto your back.
Why it maps
The mud is tori's hooking foot pinning the heel to the ground: with the support anchored, the chest-high push has nothing to do but rotate you straight over backward.