What happens, more or less, is this: “Under conditions of metabolic demand, activation of the intercostal muscles to maintain lung inflation during expiration sets in motion reflex contraction of laryngeal muscles, creating a system under pressure that lengthens the expiration phase of the breath and enhances oxygenation of the blood. Expiration against the constricted glottis produces pulses of sound.”
Translation: In the right circumstances, the sound just happens.
McCune went on to explain that tennis players often grunt when they hit a ball off their racket and that trying to stop the sound can actually hurt their game. “When you squash the grunt,” she said, “you’re having to use energy that you could have used for your stroke to suppress a vocalization.”
There is even a study that proves McCune’s point. Researchers from the University of Nebraska Omaha found that professional players increase the ball’s velocity by 3.8% if they grunt while taking their shot.