In order to understand Vago-sympathetic voice stress analysis, one needs to know that stress, when accompanied by guilt and a known consequence, will cause the body to physically react.
Everyone has experienced stress as related to deception at some point in their life. Whether being accused as a child with regards to stealing from a department store: questionable Internal Revenue Service filings or perhaps even an Infidelity issue, stress with any consequence of discovery will manifest itself demonstratively.
This stress can be overtly revealed in the form of observable reactions such as sweating, a shaky voice, dry mouth, increased body odor, runny nose, upset stomach, hives or coupled with notable body movements; i.e. body language. This overt response is commonly referred to as the "Fight or Flight Syndrome." The expression of stress however, is also more latent.
Since the Central Nervous System supports the Autonomic (Uncontrollable) branch, these latent emotions of stress will manifest in a recordable way and cannot be controlled.