Digiface USB is set up in TotalMix so that it routes Optical 1 input to Optical 1 output.
Settings are so that Clock Source = Optical 1 clock the Digiface to the incoming sample rate of 48kHz.

Now when setting back Fireface USB Settings to Clock Source = Internal; Sample Rate = 44100 Hz, I see Input Status Optical 1 = Lock, Sync, Lock, Sync etc.

I was expecting also that the sound would become garbled somehow, but actually it keeps sounding all right.

How does this work?