Brainwave entrainment happens inside the brain, and is caused by a physiological response. Upon hearing two tones of different frequencies – sent simultaneously to the left and right ears – the brain perceives a third tone based on the mathematical difference between the two frequencies.
The brain then follows along at the new frequency and produces brainwaves at the same rate of Hertz (Hz).
For example: if a 200 Hz sound frequency is sent to the left ear, and a 205 Hz to the right ear, the brain will process those two frequencies and perceive a new frequency at 5 Hz.
The brain then follows along at the new frequency (5 Hz), producing brainwaves at the same rate of Hertz (Hz). The technical term for this process is ‘frequency following response'. (Binauralbeatsmeditation)