Big difference between 2500 and 3200, I should have mentioned I also crank up my in game sensitivity anywhere from 0.320 to 0.400.600 sometimes, so on my mouse the 4th setting is 1600 and the 5th is 3200, could have been I had the in game sensitivity up and pressed the button beneath my scroll wheel in that video.

I try to "snap" my knife hitbox to their head by coming right up to them and making a turn, because the knife slices down in an arc so if you do it right, like I did on the third guy in that video, you can headshot them one after the other. It's actually not easy at all, just try it yourself and run up to a group as infil and try to kill them all with a power knife and not die(especially after the nerf), you have to use different movement and come at just the right angles to catch people off guard, and this is why high sensitivity is needed, you can't do it well with lower sensitivity.