At that time they used a Studer A80 (if memory serves) 1/2 track machine, modified, with an extra playback head that was placed before the head stack so it read the music on tape about 500mS before the playback head got it. The extra head sound was fed to the motor controller that controlled the speed of the cutting head feed motor that turned the screw that controlled the pitch depth of the grooves.

When the preview head sound was loud, the screw motor would slow down to make bigger grooves and then return to normal when the audio envelope was smaller.

That's how they optimized groove spacing before digital buffers. :-)