The voice is a non-linear instrument. Indulge me in an title . Let’s say I’m going to hammer a nail into a piece of wood. The harder I hit the nail, the further it goes into the wood until it’s in all the way. If our vocal folds are the nail, and our air flow is the hammer, the same is not true. For the voice to sound to its full potential (in volume, resonance, etc.) it must be released. If we’re straining, or forcing too much air over the vocal fold for the given range that we are in then it will not be able to release as fully as when the airflow is commensurate to range. For our voices to be full, the vocal fold must be sufficiently released (relaxed) and the surrounding soft tissues in the aural cavity must also be allowed to vibrate freely. This creates a feedback loop that creates a much louder sound than if we just try to force more air over the vocal folds. For a more technical description, check out.