The feeling in your head that comes when you take in smoke that immediately changes your state of mind. If you weren't high before the brain punch, you will be after. If you were high before the brain punch, afterwards you'll have turned into a temporary vegetable.
Right as the gravity bong's smoke entered my chest, I felt my brain crash into my skull. I immediately fell into the closet that was behind me and was asleep before i fully hit the floor. I got brain punched.