I have seen so many people who just jerk up and kick and the horse, more often then naught has too much backwards momentum and throws their head back. It scares me seeing those people that do that, just asking for the flip.
Yet you seem to have taught forward momentum and subtle cues that have nothing to do with head tossing or pulling back on the bit. Bonus points to you. Looks pretty much to be the same way my boy is asked to rear.
That's the problem I've seen too and to start with our issue was part of that. She was using it as a way to get out of work which never worked. Then when it was turned into a job she was reluctant because she had been told so many times not to do it. It was the middle mark where she was still doing it out of spite and then learning it as a command and getting confused. So I did end up pulling her up a couple times just until her feet came off the ground and then it was all her.
However for the last 4-5 months I've been focusing on the leg cues because I want her to listen to my legs, voice, and my seat when she comes up, not my hands. I mostly rear her bareback, I seem to have a better seat and leg on her and when she comes up my hands are in her mane with little contact on her face. When my legs release she comes down. She's a good listener and focuses on my voice. If she comes up before I say Lift she gets a loud "DOWN!" and she'll drop right back into set even if she hasn't come up all the way.
So now that she knows what the check on her face feels like from training back at the beginning and that hands down with contact means stand and hands up with a loose rein means "at the ready", I don't need to pull her up anymore. She just needs the feeling of the rein on and then off her neck to know she's coming up. If that makes any sense. It does when we do it.
Now her little attitude will throw in a mini rear and hold with some of my more experienced friends. She'll come up about a foot and just chill there while they sit wide eyed. It's normally right after they've trotted her a bit and I'm assuming she's saying "you think you know what you're doing, lets see if you can do this!".
I always tell people that if for any reason she starts hopping on her front end pull either side down and circle her. Puts her mind back to work.