The Blumlein monogroove on gramophone records that have stereo signal modulation incorporate two modes of modulation at the same time: engraving; and impressing. Engraving is accomplished by the lateral wiggling of the stylus, which is in response to stereo signals that are identical in amplitude, frequency, and phase. {In fact, the solenoids of each channel are signaled to with opposite polarity so that in-phase signals will be able to push-pull their agreement like two people on opposite sides of a saw, working in tandem.} Whereas, if the two channels are playing the identical signal, but the phase of the A channel signal is 180-degrees opposite that of the B channel signal, the summing of their phase disagreement will make for vertical displacement of the stylus. A soloed channel of a stereo cutting head cuts diagonally. Impressing is the groove modulation caused by the vertical wobbling that is in response to stereo signals that are out of phase. (Embossing is when something pushes up into the bottom of a workpiece from below, creating a visible protuberance when viewed from the top side. Old business cards and invitations were often embossed. Although no cutters actually emboss, some people refer to monophonic impressing as embossing.)
In this photo are impressed grooves that were being modulated by a 1,000 cycles per second, stereo signal that was 100% out of phase (sometimes referred to as "channels in opposite polarity").
A fair amount of land is left to preclude overcuts during either sudden dynamic passages, or where blank surface height variances sometimes occur (which can introduce significant offsets in the achieved groove width), but the disc computer is obviously taking note of the timing and polarity of the two signals being impressed as a monogrove of varying depth, since the more deeply impressed grooves are closer together than they could have been, without creating overcuts, had the signal's periodic pattern's timing not presented the opportune profile for nestling - the Pangaea effect - or, puzzle pieces nearly spaced. The computer happens also to be mindful of absolute polarity, based on the standard of having positive excursions start by moving the stylus to the outside of the disc, in the case of summed signals, and downwards, in the case of difference signals.