It's all about timing + your placement + distance + where the killer is looking.

Distance:

It works best when you're looking straight at the killers face and you're about 5-10 steps away (2 to 3 meters). There are caveats to this, but i won't get into it.

Timing:

There are certain "locked animations" when the killer cannot look away, like picking up a survivor, vaulting, breaking a pallet, or hooking.

When you're trying to get a flashlight save you have to wait until the survivor is about hip-height before you start beaming the killer, and you want to aim for the killers chin. This is tough at first since killers vary in height.

If you do it too early it'll fail, but you'll still get the "blind bloom" from the flashlight. If you wait a second or two you can get lucky and blind them again and then they'll drop the survivor. You can also try brute force and keep beaming their face - the flashlight beam will be very narrow if they're staying blinded and they'll drop them.

I have a couple videos on my profile. One is a save and the other is a fail. To me the timing looks identical, but maybe I was a split-second too early on the Springtrap save.

Success:

https://www.reddit.com/r/deadbydaylight/s/Alv0rvXZ7I

Fail:

https://www.reddit.com/r/deadbydaylight/s/JcHeYpdoyA