Help plz I'm stuck on chpt 5 killed the boss but can't disconnect the creatures from the brain. I keep knocking them down and standing over them, but the x contextual action button doesn't do it. What button/ method do I use to unplug them? i did it once by accident, can't do it again. Thnx!

It can be a bit tricky, because you're likely getting attacked at the same time, but you just have to find the right position to stand and the X prompt will work.
Try to separate the zombies so you can get some room away from taking damage.
I had to try a few times before I could get it to work right.

For clarity, it's the same button you use to burn enemies, not the button used to pick up ammo and open doors, etc.