Parasympathetic Nervous System... Stimulation has a inhibitory effect on heart rate with it's influence starting and ending quickly, ie. the parasympathetic nervous system has a beat by beat effect. Plus in the lungs, parasympathetic stimulation constricts the blood vessels and airways.

The sequence in which Body'Fit pH Fitness exercises are done, moves the cervical vertebrae and stimulates the parasympathetic nerves only once the heart rate has been accelerated to training level by the slower acting and longer lasting sympathetic nerves during the lumbar and thoracic series of exercises within the routine.  

The vagus nerve that runs into the chest, originates in the medulla oblongata and passes through the cervical vertebrae. The right vagal nerve predominantly affects the ventrical and the left the atrium. The neck exercises in Body'Fit pH Fitness always begin on the left side first, stimulating the left portion of the nerve slowing the atrium and reducing the speed of blood flow from the lungs in order to increase time for oxygen pick up and carbon dioxide removal.