Normal mastering is done in real time, meaning that the master tape moves at the same speed it did during the recording session and the lacquer spins at the standard 33 1/3 rpm. Half-speed mastering is when the master tape is played back at precisely half its recorded speed while the cutting lathe is similarly turned at precisely half the desired playback speed (ie 16 2/3 rpm).
When the mastering speed is halved, the power required for the cutting head is reduced by a factor of four. This reduces the load on the power amplifiers used and allows them to run in a completely linear fashion. If marketing is to be believed, the cutting stylus is able to trace the delicate groove undulations more accurately, with major improvements in frequency response, distortion and transient response. Crosstalk between channels is substantially reduced, control over groove dimensions is improved, and the ability to cut high energy passages without distortion is greatly increased. It's for this reason that half-speed mastering is the choice for "audiophile" pressings.
Sgt. Pepper's Lonely Hearts Club Band was the only Beatles album to receive the half-speed mastering treatment in Australia, first in 1982 (released in 1983) and again in 1987. Please visit the dedicated album page for more information.