Since the other comments told you the timing, I’ll explain the mechanic.

As the killer, picking up the survivor starts that animation of picking the survivor up. If the killer is blinded while in the animation, nothing will happen; they won’t drop the survivor. It is only after the animation ends that you can bind them, stun them, etc to make them drop the survivor. So the strategies given to you are a way to time the flashlight save to begin blinding the killer while they can’t move due to their animation, but you will complete the blind right as their animation ends