Tie downs are for horses to brace against, mainly in speed events and such. They are not to be used for headset, collection, etc. If your horse tosses his head or runs through your hands, a martingale of some kind (after a soundness check) could be useful. Be careful and get a professional opinion on the fit and use of a martingale before you put one on your horse as they can be damaging in the wrong hands.
Also worth noting, side reins and draw reins are not to be used when riding for 'headsetting' or getting a horse into 'frame'.
ETA: I apologize, I just reread your post. Looks like you chase cans? :) A tie down should fit so that it is not pulling the horse's nose down at all, and not so loose it could get tangled up in something. If your horse can comfortably brace against it in a run without throwing his head way up, and without having his head tied way down, that is good. Look for pictures online of barrel racers to get an idea.
I have said what I have said. I have not said what I have not said.
Last edited by RunSlideStop; 06-14-2012 at 05:18 PM.