The idea is fairly straightforward, I think. It’s about whether the black has to throw in at S11. If it’s seki as it is then nobody gets any points. If black throws in they lose one point - the captured stone.
If black gets T9 then it’s seki as it is without throwing in. Even if white tries to capture the corner by filling in 1 and 3 and then 5.
Black throws in 6 at the last moment so now white has to capture corner to live with 4 points but black is capturing four stones so they get 8 points. Thus white is -4, they aren’t gonna do that. Black isn’t going to throw in before 1,3,5 are played so it’s seki as it is after black got T9.
If white gets T9 then the calculation is different. Here is the same position but the T9 is white.
Now white can do under the stones. White captures the corner, black captures five stones and white recaptures with 11 and then 13 at T10.
Now we need to calculate the points. White lived in the corner with 4 points, black captured 5 stones, white captures two stones worth 4 points and black can make one eye there so 1 point too. That’s +4-5+4-1=+2 points for white.
Black doesn’t like that so as soon as white plays T9 they’re gonna throw in. And white has to take otherwise black takes with atari, no under the stones nonsense. This is the result. One captured stone - one point for white compared to no points in previous case.
