With the No Mix switch position, the channel is muted for summing, but is retained at the Direct Out output and can thus be recorded again.
The No Mix function is especially useful when, for example, a compressor is inserted: the compressed signal can be recorded again and assigned a level automation.
Without this function, also known as “bounce back”, signals with level automation could not be compressed, because the compressor is looped in behind the post fader control and therefore the compressor threshold could not be adjusted during level runs.