Does no one seriously know how to stab a horse? I feel like I'm fairly decent at counter cavalry, and it's really not that hard. Don't let them bump you, and if they get close, stab their horse. If they are doing that thing where they try and wait for a chance to attack holding a swing and trotting just out of reach, then hold a stab and wait. Most of the experienced cav will know not to run straight at you, that's how I know if you are good or not. If they know you see them, the good players will try and get behind you to bump slash or straight kill. That's why spin stabs on cav work well too, they don't know that you are watching them and think it's an easy kill and then when it's too late to react, you spin and kill their horse.
You want to have the horse charging straight at you when you stab it, so that the bayonet hits in the head or chest of the horse. This insta kills even heavy cav, so.
Maybe I should make a video or something since it seems to be such a difficult concept