I thought it seemed interesting to examine how it would be to do
Straightness Training with a Parelli halter 
And it was fun!!!
Normally I work with a cavesson and a curb. But a method should not only depend on tools, it is more communication between trainer and horse.
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The question in these videos is:
Dear Marijke,
I am getting ready to re-start my spanish gelding under saddle. He had been healing from a pasture injury. I have noticed he has a tendency to drop his back and not use it properly. I have started the shoulder-in on the rail and circle on the ground and he is doing well. His foot fall is supple and has good rhythm. What would be the best exercises to do for his back and neck. I do not want to ride him until i know he is ready for a rider’s weight. He is a very special and sensitive boy, and when i first started him he was just amazing. I don’t want to do anything too soon to lose his trust. Thank you for the generousity of your time and willingness to share.
Dawn
Part I
Part II
My youngest horse Prince Elmelund is learning to work at hand in trot.
He does the shoulder in, the quarter in, the renvers, the traversale and the renvers on the circle.
Of course it is work in progress, but he is developping very well.
Take a look!
Click