Why do boxers make a hissing sound when they punch?

They are linked. The result of tensing-up and breathing out sharply at the same time is a sharp sound that may take any form modulated by the boxer's mouth. What many prefer to do is keep the mouth almost closed, and force the breath out - this adds to tension, which is good momentarily on the hit. read more

When you exert energy its good to exhale, the same way weight lifters grunt when they lift heavy weight. It's the same principle for when boxers punch, they exhale when they exert force. Sometimes they don't hiss, but do other stuff like grunting. read more

Huffing while punching is sort of a golden rule which they teach in boxing. The effect of punching without exhaling is much lesser than that of while exhaling. If the boxing classes have very advanced instruments, they show you with the help of a device which calculates the power of your strength. read more

That hiss is the sound they make by exhaling. It helps regulate the breathing, having a good respiratory and oxygen level in the blood. Also, if you ever lift weights, exhaling helps with the movement, I can't explain how (hopefully someone will), therefore it can make the attacks slightly stronger. read more