when you ask the horse , through the reins, to soften to the bit , (I wouldn't necessary call it be 'on the bit', as that implies that he is driving forward with an engaged body, but is allowing the bit to control a bit his head position and is lifting his chest a bit, while reaching further underneath with his hind legs . . . so-called 'collection')
Anyway . . . So, yes, riding 'back to front' requires the hrose to understand what the 'front' is , and this is what you are working on with your rein work. I think of it as getting the horse to answer me when I pick up a bit of rein and ask, "can you soften a bit and get ready for something ?" "are you 'with' me?"
When asking a horse to soften at the poll, it is a first step in preparing him for the next thing you want him to do. Your horse is just being a good fellow by trying to respond to you and do MORE than you ask for, and that's probably becuase you are not rewarding him soon enough , so he is confused. If you want him to only soften to the bit , then you must be ready to release him when he does, even if it's only a tiny bit.
But if instead of softening, he thinks you are asking him to back (and he does so with a hard, braced neck, then LET him back up and up and up and up, but you keep holding the rein with enough contact and keep looking for him to soften. The instant he softens, release.
you do NOT reward a braced back up, but you DO REWARD a softening, whether he needed to back up and up to figure it out or not.
The softening comes first, before the back up, and if you reward that, he will know to wait for the cue from you to move. If he backs before softening, you do not need to use your leg to move him forward, you allow him to struggle a bit through it, until he figures out that SOFTENING is what you are looking for, not backing.
And, yes, your seat is all important to indicate that you ARE or ARE NOT asking for a back up.