Black Mizore fights in her sleep, so it's difficult to predict her moves.
Having dodged all of Shiro's shadows, Mizore uses him as a body pillow, and falls asleep while giving him a sleeper hold.
When Shiro reaches orgasm, she doesn't notice because she is asleep.
In other words, the sleeper hold does not loosen.
Shiro has no choice but to continue to orgasm while choking until a doctor stops by Will...