A nose-touch is a behavior where your dog literally touches their nose to something. It often starts as a nose touch to the palm of your hand presented at nose height to your dog. One of the reasons that agility enthusiasts teach a nose-touch is that it can be transferred to touching a target of some sort (like a mouse pad or yogurt lid) to enable stopped contact performance. If you can stop the nose, in theory the rest of the body stops too.
From my perspective, that is probably the least interesting application of a nose touch that I can think of, even though I think its a great behavior to train. The reason I don't like it for contacts is because I don't know many dogs who want to run as fast as they can for the opportunity to bonk their nose into the ground. I don't know many handlers who like that either, so it pretty much guarantees that at some point, you will begin rewarding a "near touch" which is where the dog bobs their head and approaches their target but doesn't actually touch it. Mostly I think this just rewards your dog for moving around in their stopped position, which I don't think is all that helpful.
So if that isn't a good reason to teach a nose-touch - what is? Here are my top 10 reasons to teach one (in no particular order):
1) have something easy to do
as a reorienting behavior for when your dog leaves their crate, front gate, or crosses any threshold into a stimulating environment (those big double doors at Argus Ranch as an example) where they could be distracted or overwhelmed.
2) have something productive to do
as a simple task for waiting in line to go into the ring or waiting for your turn in class
3) have something calming to do
when presented low or in a lie down
4) have something energizing to do
when presented high so they have to jump for it, or chase you to do it
5) have something deliberate to do
as a way to learn to deal with distractions. Can you do your nose touch when I have cookies in my other hand where you have to look away from the cookies to do it? Can you do it when another dog is running near by? Can you look away from your ball and do it?
6) have something focused to do
when there is too much going on. There's a scary photographer, and a weird noise, and some nervous dogs barking, and food smells and, and, and.... How about you and I just play a game that you know really well instead of paying attention to all this other stuff that has nothing to do with you - can you nose touch?
7) have something reinforcable to do
when you get in a training jam. Should have stopped on the last try? Trained a bit too long? Missing weave entries, blowing contacts, wandering off and don't want to end on a bad note? Lets ask you for something easy that I can reward you for and rebuild your attention and focus or confidence or just build more reinforcement history so you can try some harder stuff again.
8) Have something quick to do
to help your dog transition from 100% environmental focus (say when they are in their crate) to 100% teamwork focus. Reorient to me? Good dog. Easy nose touch? Good dog. How about the other hand? Good dog. How about recall to heel and touch at my side? Good dog. How about collect over the jump and touch at my side?...
9) have something specific to do
when teaching persistence. One nose touch? good dog. What about two in a row before you get your toy? Good dog. What about doing one for a treat? Good dog. What about doing one for the chance to play tug instead of a treat? What about working harder to do one or pushing harder with your nose?
10) have something reassuring to do
around strangers as a checkpoint to see if your dog is comfortable to approach them or not so you don't accidentally force attention on your dog when they don't want it.
Oh yeah, and it can be used as a target to teach other behaviors: Orient to side, Lining up at the start line, Down, Sit, 2o2o etc...