Enjoy your Ride!
Maybe your horse doesn’t want to slow down and you are using a stronger bit. Or perhaps he tosses his head and you’ve put him in a martingale or tie-down to keep his head down.
Either your horse doesn’t understand how to slow down, doesn’t respect your idea enough, or he’s too scared to do so.
Pulling straight back on the reins it only works to a degree. He can either grab the bit with his teeth or completely use his powerful hindquarters against your wish to slow down.
A more effective way to stop a horse is to teach them first to follow the energy level in your body, then use your seat cues to slow down, and then one rein if he doesn’t stop.
It is much harder for him to run away with his neck bent. Bending the neck also moves his hindquarters away from the Side that you are using the rein on, which makes him less powerful.
You can’t just go out and haul on one rein and expect him to stop!
The idea is to use as little pressure as necessary, but enough To get the job done.
Remember to release the bit only when he is doing what you ask, or in a positive step towards what you ask.
If at anytime you are out of control while mounted, go back to working on the ground.
If you are having issues stopping your horse you don’t need stronger bits, you need more training and preparation and it starts with the above and a solid ground work program.