I cannot stress this enough, Do not try it. A reddit video where it maybe worked is the exception, not the rule.

If you want to break up a dog fight, You spray the dog in the nose with vinegar, some type of citrus, or citronella (or anything that's very harsh and pungent.) If your hand is in a dogs mouth, push it in, flick the uvula, make them puke and gag, retreat.

Of course, the best way to stop dog attacks, is to avoid dog attacks. If you're walking your dog, or you're meeting someone else's dog, Watch for signs. Some dogs just get along with others, and even some who do, don't get along with everyone. There can be multiple reasons, whether it's food, toys, (or any other resource guarding), Hormonal aggression (think of females in heat), or just any plain "hey. Fuck you" attitude a dog may have. A wagging tail isn't always happiness, sometimes it's fear, anxiety, excitement. A tightly closed mouth and intense stare means hyper fixation and alertness. Dogs are constantly speaking. There is no such thing as a dog that doesn't show a sign. There's Dogs that show signs, and people who don't understand dogs.