The state of heart coherence is an archaic heritage from our stone-age ancestors. It corresponds to a state of maximum vigilance, perfect adaptation and maximum perception to the environment. If we look at the animal world and the continual battle between prey and predators we can easily see the mechanism at work. In the African savanna for example, an antelope is busy grazing, he looks relaxed, from time to time he lifts his head and scrutes the territory around him to see if he can perceive a predator. If the predator is outside his perimeter of danger, he continues to graze. If we take his heartbeat pattern at this moment, we find that his heart rate is gradually increasing and decreasing in speed - he is in coherence with his environment. The moment the predator moves into his safety zone, the animal’s nervous system instantly changes balance and the animal springs into action to flee. He knows he cannnot fight! the hormonal changes instantly produce adrenaline (energy) and cortisol (a booster) and also canalizes his energy reserves to his muscles and brain away from his vital organs. His heartbeat is chaotic and accelerates and decelerates randomly. If he successfully escapes his enemy, there is a period of calming down and reregulating  internal balance to eliminate excess cortisol and adrenaline.
The most efficient way to do this is when his heart beats coherently as he is in this state aware and vigilant. Physiologically the ANS is in balance between its two branches. The body is not in relaxation as the heartbeat is not extremely slow. The sinusoidal curve helps the heart to accelerate to high speeds without damage. The greater the variability, the easier it is to achieve fast heartbeats needed for survival!
This state corresponds to maximum wellness and peak performance.