Using just my reins today, I closed my hands at a halt; she lowers her head and looks on the vertical, expecting to stretch down (only rein pressure at a halt usually was our time to do that). I hold the pressure and wait. She licks and chews, then tucks in more so I have to move my hands back to keep the same pressure. She jerks her nose back up in frustration after a second and scoots one leg back, so I release then. I kept trying this, and found that she stopped lowering her head and licking the bit when I pressured, instead ignoring my hands and sometimes taking a one small step back, but only with her nose out and clearly not accepting the bit at all.
So it looks like same pressure until step back only results in her becoming unresponsive and tenser. Some months ago I would practice closing my hands and waiting for her to lower her nose and get on the vertical, releasing every time she moved the right way any slight amount. So it would seem that if I just do that again, she will only get better at going on the vertical with rein pressure and stretching out from halt, but nothing to do with backing!
How do you teach a horse to back up on rein cue only, accepting the bit as they go?