Effect punching is Anaerobic ('with out air') breathing as if you are running breaks your rythem down. Your ment to be exploding when you throw a combo... breathing normaly as you punch also relaxes the intercostale and Abdominal Muscles which give some protection from counters to the body if they are tense (when you breath out).

Once punching becomes even more natural you'll stop 'hissing' and do a shouting sort of thing with your combos... :P