Why do boxers make a hissing sound when they punch?

Striking power is increased by both tensing-up and by breathing out on the hit.

Fighters have to breathe. This means both in and out. Breathing in is as important as breathing out, since without one you can’t have the other. It follows that you need to be breathing out when you’re striking, and breathing in when you’re not striking.

That is the basic reasoning behind it; breathing out on the hit is important (more so with harder shots), but breathing in at the right time is the flip side of the coin. You can’t have one without the other. Stamina is inextricably linked with this.

Power increase

• Strike power is increased by two things: tension and exhale. 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. Therefore a common result is an ‘eesh’ from between the teeth.
• Almost all strikes benefit from tension/exhale, so it is common to exhale on most hits - a light breath on a light hit, and a heavy breath on a hard hit. The rare exceptions that don’t require tension still benefit from the exhale.
• In a fast combination there may be a continual light exhale through clamped teeth, or just one on the last hit.
• The fighter may choose not to exhale at all on some shots, especially a quick jab. Or, exhale sharply but through the nose, on these light hits.
• Taking a hard shot in the body - absorbing the hit - benefits from the increase in strength the exhale provides. This extends to taking a hard shot on a block, as the limbs and body can be fortified. You can see this with a Thai boxer slamming a hard shin kick into the opponent’s body, who takes it on a 3-point outer block with a strong tense/exhale to add resilience. You see it when a boxer drives in a hard hook to the body, and the opponent chooses to take it and counter at the same time, maybe taking some of it on a low cover, and using a strong breath.
• The modern explanation for this involves ‘valsalva’. If this doesn’t mean anything to you, please look it up - thanks.

Breathing and endurance
Correct breathing facilitates stamina. Incorrect breathing saps stamina.

Endurance is learned by fighters in fights, and part of that process is learning how to breathe. An experienced fighter is good at fighting for many reasons, and at least half of it is they have learned how to control their body. It is surprisingly important, and also something that non-fighters cannot learn and cannot appreciate. Controlling the body while hitting, being hit hard, wrestling, getting thrown and wrestling for superiority is not easy - indeed only fighters know how to do it, through experience.

Without this control, endurance and durability reduce dramatically. Breath control is fundamental to endurance, and to power delivery.

Breathe in and out
Breathing needs to be in & out or the boxer falls over - this may sound funny, but it is important - coaches will know from experience with novices that this point is not always appreciated! The boxer must also tense and breath out on a hit. Breathing benefits from timing. Out on the hit or the block/absorb, in at any other time.

Tension and breath, fluidity and breath are indivisible. Both strength and fluidity come from breath control. A fighter needs both power and fluidity - some of each, at the right time.

Stiffness, rigidity, lack of endurance, then exhaustion and lack of power - all come from poor breath control. A fighter needs to learn how timing, breath, power and fluidity are linked. We teach it by simple methods of doing and learning, since these concepts are probably beyond even junior coaches never mind the fighters. A fighter can ‘do’ correctly, when taught correctly, then they will learn automatically.

Fluidity
I avoided the use of the word ‘soft’ or ‘softness’ - it can be misunderstood; fluidity was used instead. A boxer needs to be smooth and flowing, in order to hit hard. Relaxed, if you prefer. Boxing is opposite in many respects to Shotokan karate, and in those particular aspects is far closer to Tai Chi. It sits about halfway between the two. I don’t usually bring that up as it tends to confuse people - but when you look at breath control, you will begin to understand why.

The best way to box is with a relaxed, fluid, loose mindset. This way produces the most freedom of movement and leads to the most power in the punches: we relax and move smoothly; the punch launches with swift acceleration made possible by relaxation; and then at impact we finally employ tension, breath and hardness, momentarily. For us, movement and striking are inextricably linked. Fluid movement requires relaxation. Breathing correctly helps to create both fluid movement and punch power.

Most of the stuff above is only of interest to senior coaches, there is no need for a fighter to know any of it. Just breath the right way at the right time, and all else falls into place.