I've copied the top speed runners technique's and they get G3 to stay down so much longer than I can would love someone to explain how it works

Use the knife and make sure you are on 120 fps as the damage is tied to your frame rate

thanks Alfie! but if you read my post I've mentioned that I copied the top speed runners so I used the knife and 120 fps but I still don't manage to get G3 to stay down consistently and don't understand how it works

share a video of what you're doing then we can tell you

By any chance do you mean this pattern?

yes this is the pattern i was talking about, ive tried on some different saves and some he stays down for long others he gets back up like ur vid the only thing that makes sense for me is that its a DA thing but i have no clue for sure

There are 2 reasons he would stand up early, either the DA is very low, so you do too much damage too fast, or, you accidentally knife his exposed chest eyes through his torso. Any damage with the knife to the eyes in front will make him start standing up. It's way easier to get this meme on Leon because his knife slash goes a bit deeper. You avoid this by either backing up a bit, or staring straight at the ground while knifing his back. DA for this fight with most recent strats should be around 6800 on CAS and LAS.

Edit: Never mind the DA thing actually, after testing it I'm certain it's just the slashing of the chest eyes that triggers him to get up.