Vocals removed and then put back in at a frequency 33 cents sharp of the backing, or roughly one third of a fret on a guitar.
This corrects for the superbugg-cockwombleⓅ effect where medium to loud headphones depress the preception of pitch by 33 cents, roughly minus 5hz at middle C, and singers traclk that perfectly - flat!
Listen to this then compare to the version courtesy of AndrewMwah https://ourtube.co.uk/video/57555