Using your finger to press into her chest should be a backup aid to the primary aid , which is either pulling back on the leadline or the rein (even if she is on the ground) If you only use your finger to teach backup, how would she know that the rein means back up in the saddle.
Of course, as above posters said, you dont' really pull her back but rather close the front door and then ask for Movement. You can in the beginning add a little more backward "suggestion" with the bit and when you feel her rock her body backward, it's her asking, "do you mean this?". That's when you release a wee bit, but don't throw open the front door.
Also, you THINK back. Imagin you are the horse and you need to move your four feet backward, one at a time. Really think "I am going to walk backward" you'd be amazed at what a difference it makes if you really think about the movement you are asking the horse to do. You will subconciously do this movement with your body and the horse will follow you.