In the good old days they used to call it "sacking out". It consisted of tying your horse to a post and rubbing it all over with a gunny sack until it stood for it. Generally it was a one time thing that happened prior to chucking the kack on his back and cinching it down. Today desensitizing is a process that in some instances goes on throughout the horse's entire life. There is a rainbow of fruit flavors available to choose from when you decide how or if you are going to incorporate this tool into your repertoire with your horse.
The science behind desensitizing is sound. Horses are prey animals. They have a well ingrained flight or fight response that has allowed them to survive in the natural habitat for thousands of years. Their instincts tell them that if you see something that might eat you, even if you aren't sure it's probably best to get out of there quickly and then maybe reassess. If you can't run, you better start fighting. Desensitizing is supposed to teach a horse to stop and assess first prior to running and fighting.
Desensitizing today is largely done the same as it was years ago with the exception that you don't tie the horse. You leave them an out so that they don't feel trapped and don't end up kicking in the fight response. If your horse always has an opportunity to leave the scary object, even if it's just running around you in a circle they will eventually stop to reassess the situation and then accept it. You can use any object to desensitize but most practitioners of natural horsemanship will use a stick and string (carrot stick or handy stick) or a flag or a coiled lariat. While holding the horse on a lead line you introduce the object by moving it all around and over the horse's body until he can stand there and accept it.
The variation in method and ideology comes in when we start discussing just how much desensitizing you need to do. Here is where the rainbow of fruit flavors comes in.
Maximum Desensitizing: These are the folks that have a regimen of desensitizing that they do with their horses each and every time that they work with them. Typically this is incorporated as part of the groundwork routine but can also be done from the saddle. You'll often hear this mantra from folks in this camp, "For every sensitizing exercise there is a desensitizing exercise". So you will first teach your horse to move off of pressure or stimulation then teach it to ignore the stimulation. It is up to the horse to read your body language and therefore your intention before deciding if it is supposed to move or go. It is also often used as a sort of cool down after a rigorous sensitizing exercise. When you start your desensitizing the horse is supposed to just cock a leg and zone out. It's his cue to stand quietly. Defendants of this process are the ones that you will often see standing on the back of their horses wielding a chainsaw or leaf blower while the horse stands quietly with a leg cocked. If you are a horse person that is particularly worried about your horse spooking at different stimuli this looks like a dang good deal. Opponents of this type of desensitizing worry that you are ruining the horses natural sensitivity to stimuli that makes it such a valuable partner. How can a horse be sensitive to your lightest cue while also being dead to all outside stimuli?
Moderate Desensitizing: Obviously this form of desensitizing falls somewhere right in the middle. You will often pair desensitizing and sensitizing exercises with your horse and may even repeat the desensitizing exercises daily. The difference is that once the horse is good and standing for the desensitizing you move on rather than belabor the point. There may even come a time in the horse's career when you stop doing the desensitizing unless a problem with a specific object arises and then you always have the desensitizing to fall back on when needed.
Minimal desensitizing: These are the folks that really can't even abide the word and will often choose a different term for what they do to "check out" the horse or make sure he is okay with stuff. This type of desensitizing is much less regimented. There aren't any desensitizing exercises you do with your horse, you just make sure he's okay with stuff. For example, you horse is worried about fly spray. You would just keep quietly fly spraying them until they quit freaking out. You would do this as needed with the horse. Or your horse is afraid of your hat. You would calmly show the horse your hat until he was okay with it. It's really just good basic horsemanship and common sense. The idea being we aren't trying to make a horse okay with everything in the world so that you can carry around an inflatable boat on the top of your horse if needed. We are trying to teach the horse that when we are there with you you don't need to be afraid because if I say it's okay, it is. You are making a pact to the horse that says, "I promise to take care of you and when we are together I'll help you watch out for scary or dangerous stuff". The horse learns not so much to tune stimuli out but to be okay with it because he has trust in you, his herd mate that you aren't going to be in trouble.
Even as I write this it seems silly to me that there is any debate about this, but believe me folks from the different camps can get down right touchy about this. The minimal desensitizers believe the maximum desensitizers are dulling their horses and making dead unthinking horses that nobody would want to ride and the maximum desensitizers think the minimal desensitizers are just "cowboys" making crack heads that are prone to buck or run off at the least amount of outside stimuli.
Let me tell you about my personal journey with desensitizing. I started out in the maximum desensitizing camp. It made perfect sense to me at first. Who wouldn't want a horse that is 100% okay with bombs going off and flags and fireworks. I faithfully did my desensitizing with my stick and string or lead rope each and every time I did groundwork before getting on my horse. I got my horse to the point that I could walk all around him while he stood sleeping as I whacked that thing on the ground as hard as I could. What I noticed is that it worked great in that context. What it didn't do is transfer to the larger outside world. I found that my horse was either completely sensitized jumping to my least cue or completely desensitized sleeping while I made a ruckus around him. I definitely could have gotten on my horse with a leaf blower and stood there blowing away while he slept. But only if I had done my running around ground work and got him tired first. It didn't help him to just trust that I would keep him safe in all situations because it was an isolated exercise. When it happened after we had done some good running around in the arena he totally understood that it was time to stand and sleep. If I just walked out and caught him and he saw something scary there was no trust. Because I have a hot sensitive horse I didn't ever end up with a dull unresponsive horse but I didn't ever get a horse that was completely with me all the time either.
So, with my next 2 horses that I started I went somewhere in the middle. It worked better, I think, but especially with the little quiet 3 year old Morgan that had a tendency towards laziness I noticed that the more I desensitized the more it took to get him to move at all. He loved desensitizing. He thought it was right up there with a good grooming. Once I realized that if I kept at it I was going to need spurs to get him to move at all I quit. He wasn't ever really worried about stuff anyway, why continue with it as part of our daily routine?
A mistake that I see a lot of folks make with desensitizing and one that seems to create freak out moments is forgetting to desensitize a horse while he is in motion. You'd be surprised at how a horse that is standing there completely dozing while you wave a flag around can turn into a nut case when you start moving that flag around while you are riding them or while they are moving around you in a circle. When their feet are already moving that flight response just seems to be that much closer to the surface. So, anything you desensitize your horse to should be done standing still first and then also while moving.
A great example of this happened yesterday with the Moony and the blue tarp. He's spent some time with blue tarp being rubbed on him from the ground and he'll lope over the thing if it's laying in the arena. Yesterday I decided to practice dragging the tarp while we were moving. I picked it up off the rail of the arena and he was a little alert but okay. I rubbed it on him, no problem. Then we started to move off at a walk. Instant anxiety. So it took some time moving off slowly in a circle dragging the tarp for him to be okay with that. I repeated it on both sides and eventually I was able to carry it like a cape flapping out behind us.
So, I wouldn't presume to tell you what level of desensitizing you should be doing with your horse. I think like most things in horsemanship it is a personal decision. But I do think you should be informed when you make that decision and I do think you want SOME level of desensitizing. I also think that having a horse that is okay with lots of different stimuli starts with having a horse that is okay with you. Build that trust with your horse and don't let him down and he'll be much more willing to believe you when you say, "It's alright."