I see a lot of boxers do this in training, ranging from the ferocious (Hatton) to the slightly gay (Khan). Is there any tangible benefit to this and do boxers do this in bouts or is it just a training thing?
Just to exhale from your core when you punch which keeps the muscles clenched/more protection for body shot counters But its unnecessary for your "exhaling" noises are louder then the shots themselves...
I personally tend to hold my breath if I don't consciously exhale loudly with every punch. I guess as long as you remember to breath you're fine. To each his own; whatever works.
I either do a clenched teeth "hiss" to a more vocal "ha" or "heh". it doesn't matter as long as exhalation occurs with the punch to help maximize muscle contraction right before or at punch connect. The rest is just preferential. Also it's important to also try not and open the mouth very wide in the process. That could lead to an injury to the jaw should a punch land while the mouth is open. Whatever it takes to prevent from your breath being held. Also, the flipside is to avoid emptying the lungs with each punch thrown. Breath control when throwing punches should allow you to breathe while throwing but not overdo it and leave yourself breathless. This tires you out as now you'll have to inhale deeply prior do any other explosive actions(such as throwing a combination after the punch thrown or fielding a bodyshot)
I remember watching footage of one of Muhammad Ali's fights with Floyd Patterson(the one where Ali was cruel and tortured Patterson with his jab for the whole fight) The one distinct thing I remember hearing in the fight was the loud "hoot!" sound Ali made with each jab. He really truly was throwing them extra hard punish Patterson for things he said before the fight. I'm not sure what brought about me mentioning this lol.
I hiss the air out through my gum shield with my teeth clenched. When tired I tend to open my mouth and it becomes more like Khans or Hattons.
Deep breathing in rhythm to your movements(i.e. boxing) provides more power to your technique, stability and balance of your body, and allows for added growth hormone excretion, and delays fatigue.