I have always used hand signals with my dogs and now that my dogs are getting up there in age I realize how important signalling is. I have noticed of late that my girl Tilley is getting a little hard of hearing. Just yesterday I was getting ready to take the poodles to the beach and I couldn't find Tilley. I finally located her in the family room where it is the coolest during the day. She was completely crashed and did not hear me calling her. So I clapped my hands once and she was awake, then motioned with my hand for her to come and she did but slowly. I am again reminded that she is not getting any younger.
I have always taught hand signals with verbal cues and usually a dog learns a hand signal before they learn the verbal cue for a behavior. Dogs watch, its what they do so associating a hand signal to an action is very easy. A couple of repetitions and they've got it, plus using hand signals teaches your dog to watch you more often. I love practicing hand signal races with the three of my dogs, the first one who does the behavior wins and gets the treat. Believe me they start paying really close attention when they've missed a signal or two and loose the treat.
You can teach your dog a hand signal for whatever behavior you like, I have all the basics like sit, down, come, stay, heel, go to heel, swing, and their release, plus all of their tricks and many many more. The important part of signal training is that your signals are clear and clearly different from each other. So each signal should look very different from another. Usually you start out with a very exaggerated signal which becomes shortened with time and experience. So a down may have started out as a whole arm motion from up in the air to pointing to the ground but over time your dog knows it as a simple finger pointing to the ground.
You may even signal to your dog and you don't know it. I love watching other people work with their dogs to see just what going on with their signalling and then explaining to them how they are using their body to signal. They never believe me until I tell them to stand completely still, which can be tough in itself and using their hand only to signal to their dog. Almost always the dog stands there confused because although the hand signal looks familiar it is missing the body movement cue..
You can dissect your signalling by yourself to see just how much signalling you are giving your dog. Just stand perfectly still and ask you dog to do something they know how to do without using any signals. If they look confused then chances are you are using signals from your body without knowing it. Down is a very common one, most people tend to bend down when they tell their dog to "down." Try asking them to down and standing completely upright and you' ll know if you are using your body.
If you would like to teach your dog hand signals you can start anytime, you can use beahaviors your dog already knows and new ones. Just choose the signal first and make sure it is something you and your dog can clearly remember and use it repetitively and your dog will learn it in no time. Hand signalling is very impressive as well.