A survivor being hooked goes through stages: the first hook, then a second stage where they must struggle, and finally death. A survivor who is rescued and re-hooked resumes from where they left off, so hook stages accumulate across the whole trial.
Counting stages rather than kills gives a much finer measure of how a trial actually went. A killer who hooked every survivor twice but secured no kills applied far more pressure than the raw kill count suggests.
DBD Arena uses total hook stages as the win condition for most killers in ranked 4v1 - each survivor reports their own stages, and the threshold depends on the tier of the killer that was played. Stronger killers are held to kills instead, because for them hooks alone are too easy a bar.