She uses a snaffle, and her aids are simply the word 'back' and occasionally kicking, altough I don't like to kick since it confuses her and she tries to move forward. I usually lightly pull on th reins, never hard since she's really sensitive. She does know leg commands, but I wouldn't know how to cue her for the back.
The rein back is a 'forward' movement. It goes, leg on, lighten seat to not block the horse's back, and 'close' the hand...not pull back.
She's backing with two hands because you're hurting her mouth. She won't back with the reins in one hand because that changes how the snaffle puts pressure in her mouth, which she can stand.
Back to basics and learn to back her w/o pulling on the reins.
Step 1: The halt must be FORWARD so that the horse halts with engagement.
Step 2: Leg on and ask for FORWARD
Step 3: Simultaneously lighten the seat to unload the horse's back
Step 4: Also simultaneously, 'catch' the FORWARD energy your leg aids generated and the horse will back up. The moment the horse thinks about backing you soften the hand. It is the RELEASE that cues the horse back, NOT a pull and NOT the 'catching of energy'.
Step 5: You may have to apply the leg aid again to tell the horse to keep stepping back for as many steps as you'd like....preferably no more than 3 steps as the back is very hard on the body.
If the horse raises it's head, it can NOT back up because the back is locked and therefore the hind legs are locked.
Do NOT force the horse's head down as that only teaches the horse how to tense and brace.
Halt the horse with engagement and the head will be where it should be and the horse will very easily back up when the aids are applied.