A light detector usually behaves like a ruler: the more energy that arrives, the higher the reading. But if you rock it harder, it stops being a ruler. The pulse can stretch, change shape, and with enough energy — jump into a new stable state, like a swing that flips over the top.
Now the detector can work like a switch: it ignores weak pushes, but a strong one flips it over, and it stays in the new position. This mode helps catch single photons against background noise and search for rare events.
When latching, the detector remains sensitive to new pulses, but its noise is noticeably reduced — the system seems to 'calm down' on the new branch.