Now, I am not a western pleasure trainer. I simply train good horses that are calm, consistent, and controlled. My Dad trained show horses back in the day before the peanut roller fad and he trained his share of champion WP horses and what he learned riding them is what he passed on down to me.
I don't like a horse to creep along like he's bordering on death with his nose hanging between his knees, but I do like for a horse to go along easy with their head level and relaxed. What this post is about is describing how I acheive that collection and cadence on a normal riding horse.
The first and most important thing is to get the horse moving with impulsion. If you start trying to really control their speed before they are comfortable moving out, then you'll end up with a confused horse that has no idea what collection is. Get them moving soft and flat at all 3 gaits before you ever consider adding cruise control.
The way that I get them flat is to "lope them until their head drops". Some horses take longer than others to realize this, but sooner or later, they all do. What I do is work them in circles and lope them on a loose rein until they realize that speed is going to get them nowhere. What I do is use one rein to control their speed a bit. If they start charging around at a dead run (which does sometimes happen), then I just pick up the inside rein and spiral their circle down to the point that they have to slow down to keep their balance. Once they slow down, then I slowly spiral them back out into the bigger circle. If they start speeding up again, I spiral them back down.
Eventually, their topline will flatten out and they will slow down on their own. When they are consistently picking up and maintaining that flat, easy gait each time you ask for it, then you are ready to start working on controlling that speed.
Now, this can either be done in an arena while working circles or out on the trail. I prefer to do both just to keep the horse well rounded and keep them from getting too set in working in just one environment.
What I do is start out at the walk. I'll let them free walk on a loose rein for 4-6 strides, then I'll ask for the stop and immediately ask for 3-4 steps of a backup. When you ask them to back up, you need to make sure to ask them to stay round and supple. If they brace or hollow out, then keep the pressure and keep asking for the backup until they do soften and round up the way that they should. After I've gotten a couple of really nice backward steps out of them, I'll keep light pressure on the reins but allow them to stop moving their feet. I'll hold them there for just a minute in the rounded and level frame. If they seem willing to hold it by themselves, I'll let the pressure off the reins and only bring it back up if they change their head position.
I'll do the walk/stop/back up/hold/sit routine over and over and over until I can feel them looking for that stop. What it feels like is a small hesitation with each step. Once they are consistently doing a controlled cadence and speed at the walk, then I move up to the trot and start over; trot/stop/back/hold/sit/trot/stop/back/hold/sit, etc.
Once they are solid at the trot, then I move up to the lope and start over again. It may only take a couple of sessions at each gait or it may take a couple of weeks of sessions at each gait before the horse starts to get really consistent.
Once the horse has begun to show consistency in a given gait, then I'll start lengthening the stretch I allow them to move before asking for the stop. That may be 5 strides, it may be 50, the horse will learn and get consistent in his own time. When he's consistent, I allow him to just keep on going and the only time I stop/back/hold is when he changes his frame or cadence; raises his head, hollows out, speeds up, etc.
What this does is gets a horse to constantly think "stopstopstopstop", it shortens their stride, slows their gaits, gets them working off their hind end, keeps their headset level, and teaches them to be consistent in their gaits. A horse doesn't run around at mach 3 with his head in the air if he's looking for a stop and he's been taught to stop properly.
The most important word here is properly. In order for this method to be successful, you have to get the horse working properly and using his body properly first. If the horse is moving forward hollow and strung out with his head in the air and he's moving backward hollow and strung out with his head in the air, then all this will do is get him stopping hollow and strung out with his head in the air.
Now, another thing to mention is that you don't necessarily have to do this at a walk, especially if you are like me and prefer a horse that will really walk out and cover some country. You can work it starting at the trot, but it usually takes a bit longer for them to get consistent if you don't start at the walk first. What I normally do is teach this at all 3 gaits and then go back and teach them to do that nice running walk. That way, you have the slow walk when you want it, the fast walk when you want it, and still have the control at the other gaits as well.
It can be really nice to have a horse willing to poke along on a leisurely trail ride with friends then immediately step up the pace and cover country without changing gaits. My main horse that I ride right now can reach out and walk and then I can ask for an easy jog and he doesn't change ground speed at all, just maintains that speed in a different gait. I really like that because his jog is really smooth and sometimes I want to ride it instead of the running walk.
Anyway, that's my rundown of how to get a horse "working with his brakes on".