You can do this by turning tempo on by pressing the encoder. Once you do you'll see the tempo LED flashing to let you know that tempo is on. With tempo on, when you turn the speed knob you'll the the Ringmod speed will quantize to certain Hz values (e.g. 440 Hz, 466 Hz, 494 Hz) Each one of these values is a note in the chromatic scale (e.g. A, A#, B, respectively).
The benefit to this is that it improves the chances of getting a musical harmonic relationship between what you're playing and the output of the ring modulator. The best thing to do is switch tempo mode on turn the speed knob and experiment.