Dog training question?

When I went to my dog's obedience cl, my instructor told me how to make my dog be quiet. He's a chihuahua, so he's extremely aggressive. She told me several things that work, but there's one that works the best. You can hold his mouth closed with your fist, make him look you in the eye, and tell him firmly, "quiet." If he keeps barking or growling, the other thing you can do is make a fist and bop him under the chin and tell him firmly, "quiet." If he keeps being vocal, and this is what works the best, grab him by the scruff of his neck (where his mother would grab him) and shake him by it until he lets out a soft cry. Tell him "quiet," but when he does get quiet, tell him "good quiet" in a happy voice. When he's vocal again, shake him again. His mother would do it in the wild and he understands that kind of behavior. He understand that you're telling him he's doing something wrong.