Backing a horse can be as easy as squeezing a tube of toothpaste from the bottom.
Envision squeezing the tube, as the paste moves forward and almost squirts out the end, you "block" it gently, but firmly...giving the horse a moment to "feel" where the release is...it is back...not forward or lateral (misunderstanding or evasion)...but back. The instant you recognize that the horse recognizes where the release is and gives any indication of it (even if just a softening in the poll and the movement of one leg one step), relax and acknowledge the effort. The effort will get more and more pronounced, until it is a beautiful back. It will be straight, depending on your proper use of seats, legs and hands.
You can get you horse to back off of proper seat and leg cues used in conjunction with nice soft, but "feeling" hands.
No need to emote, "saw" the reins or otherwise indicate that it's the rider not the horse with the problem. I teach this with a halter or simple snaffle bit, (used as it is intended).
