The training bit does not just rely on the roof of the mouth. It also distributes control on a nose band and at the poll at the top of the head ALL at the same time. That is why it is more effective than anything else on the market right now.
Frank Evans spent 50 years developing these...
Take a look at this video called NIGHTMARE PONY
My girl is also barn sour. I had difficulty getting her out of the paddock into the field too... And she didnt want to move, same situation.
I got her a Mikmar Training bit and it was like magic. I could FEEL the difference in her...