Some dogs are better at following hand signals than in learning verbal commands. Actually, you can train your dog without ever using verbal commands if you don’t want to.
Body language is a dog’s first language. The first part of training a dog is to teach them that the words mean something. Many dogs understand that concept, but others can find it hard to grasp. That doesn’t mean they’re stupid-- think of them like a human who’s good at math, but sucks at verbal skills.
Dogs that learn verbal commands are often responding more to tone. If you have a dog, run him through his commands and listen to yourself as you’re giving them. You’ll likely notice that you say each command in a different way. People usually give the command “sit” drawing the word out a bit. “Down” is delivered in more of a abrupt tone. “Come” is usually given in a sort of breathless, high-pitched voice.
Here’s another experiment. Give the dogs the commands in another language. Chances are, they’ll obey it. I used to impress my friends with my “multi-lingual dog”, giving her commands in German and French, or whatever language I had briefly perused for the correct words. The dog would obey them because I was delivering the commands in the same tone of voice that I would use if I was speaking English.
When I train a dog, I use both a verbal command and a hand signal. For example, the command for “down” is to lower my hand, palm-down toward the floor. When I give the command “sit,” I fold my ring and pinky finger into my palm and lift upwards.
At first, you “trick” the dog into doing the behavior. Give the verbal or hand signal command for “sit” and then put a treat in your hand and move it back behind the dog’s head. The dog will try to follow the treat with his eyes, lose his balance when his head goes too far back and automatically sit. Praise lavishly, saying, "Good sit! Good sit." “Trick” him a couple more times and then give the command. Look expectantly at the dog. Most of the time, when a dog doesn’t know what to do, but he wants to please you, he’ll sit. When he does it, praise the bejesus out of him. Lather, rinse and repeat.