There is virtually no signal at all if an FM-X operator uses the Res2 waveform with a Skirt setting of 7 and Resonance of 0.

It doesn't even appear on the screen in virtual analyzer. Using a Resonance of 1 shows 10 partials but all are below -57db.

But playing and holding C3 while toggling resonance to 1 and then back to 0 shows 17 partials with the 1st 6 above -45db as long as you hold the key down.

Then if you repress the key there is no display because the new Key On starts with a fresh wave.

The toggling of resonance with the key down appears to be phase shifting the new signal compared to the low signal which 'cancels out' the original 180 degree phase shift and produces a usable signal.

1. Init Normal (FM-X)

2. Set op 8 to Res2 and Skirt 7

3. Press/hold C3 - no visible spectrum on visual analyzer but very low audible signal

4. While holding C3 change resonance to 1 and spectrum appears

5. Still holding C3 change resonance back to 0 and it's spectrum appears

6. Release and repress C3 and there is no visible spectrum

The above results are similar to the total cancellation shown in a demo in another thread:

https://yamahasynth.com/community/montage-series-synthesizers/montage-m-the-importance-of-phase-in-fm-programming-or-how-to-make-a-sound-disappear/#post-128573

The skirt setting 7 on the Res2 waveform appears to be producing a nearly 100% phase-cancelled signal which, by itself, would seem to be rather unusable. But even a small disturbance of that waveform shifts the phase enough to make a signal burst onto the scene.