I have read your book and I have a quick question for you. I am riding my four year-old in an O-ring Snaffle but I would like to move him to a different bit. At this stage of training what would you recommend I try on him?
Before you transition your horse into a shank bridle, I would suggest you spend some time in the Cowboy German Martingale, outlined in my book, and the Reinsman #744 Six Bit. The Six Bit will introduce curb pressure while the martingale will help eliminate the escape routes some horses use to avoid moving forward into the bit.
After that initial step, I would then suggest introducing a bit with a shank in order to work towards finishing your horse in the bridle. The leverage brought on by the length of shank, purchase and curb will give you added control as needed. A gag type bit will offer some soft warning before the bit and curb comes tight. The Tender Touch works nicely as a transition bit.
The transition from the snaffle with no shank to the bridle with shank is a critical step in the training process. If the transition phase is skipped, often times horses lose control as speed and pressure is added. The older the horse becomes before this transition process takes place, the more difficult it is to accomplish. Bridle changes can be introduced in the confines of a round pen. This will allow the horse to learn the feel of the equipment without the influence of the rider on his back.
I hope this helps, keep us posted on your results.