A law of physics
Entrainment is the predisposition that two oscilliating bodies have whereby they will lock into phase so they vibrate in harmony with another.
Entrainment occurs when two or more rhythmic cycles become synchronized. The principle is universal and can be found in many areas of science, including chemistry, neurology, biology , medicine and astronomy.
When the brain is given stimulus through the ears or other senses it emits and electrical charge and when the brain is presented with a repeating stimulus is responds by synchronising these electrical cycles to the same rhythm- commonly known as frequency following response- FFR. With the use of instruments we can stimulate brainwave frequencies associated with wellbeing.
Psychoneuroimmunology is the study of the central nervous system and the immune system and how brainwave states affect this.