u/Cereal-KilIer

I just learned about this experiment and it's blowing my mind. I kind of understand what the idea of the whole experiment is but I was wondering how they were able to decide the detection after the photon had passed? if there was any kind of detection before but it was "turned on" after the fact wouldn't it still count as detection from the photon's perspective? maybe I am missing / not understanding something but if anyone could explain i'd greatly appreciate it.