there are three requirements needed for the counter to go up:
1. you must be close (7 feet-ish) to the unhooked survivor. (i like to heal them so they don't move and i know they'll stay put until the killer can catch up)
2. you must be hit within 7 seconds (approximating) of when the survivor was unhooked. it's a very short window.
3. you must get protection points shown on the left side of the screen.
be close to a chase. i like to use bond (the only perk worth anything, imo), and when they are down you must be VERY close. by the time they are hooked you need to be within several feet or on the opposite side of the wall because the killer will then begin to move away after they're hooked. IMMEDIATELY run for the hook because you need him to come back and the time countdown will have started.
typically i will get hit, run to something super close to block my view and watch for the unhooked to be caught again. once hooked again i run and unhook yet again though i will be downed that time. (deliverance
get credit for getting hit while mid-unhook, but it's flaky.you can
still get credit if they down the unhooked survivor first. let them still hit you after.you must
be the first hit after the unhook (barring the survivor that was hooked). you can't be the second.
it seems killer special attacks and the huntress' hatchets will not
count as a 'hit.'
the achievement tracker is slow sometimes and can take up to a full 10 seconds to reflect the tick.
4% of the time you will not get credit though everything was done correctly.
i'm sure this was an attempt to cut down on tunneling, but with the low amount of seconds one actually has it makes it much worse for the team because the killer must essentially be with you during the unhook.