The MOD inputs route an external signal to an array of 16 filters at the same frequencies of the main filter array that shapes the sounds you patch to the MAIN input.
However, the product of these 16 modulation filters is not audible, since their purpose is to generate as many envelopes according to the intensity of the signal passing through each band. The envelopes then control the amplitude of the main 16 bands. This function is often referred to as ‘spectral transferring.’
If you patch an LFO to the modulation inputs you will not hear anything, since its frequency will be below the cutoff of the lowest filter.
If you patch a sawtooth or a square LFO, however, FUMANA will detect the transients, thus providing interesting, snappy sounds.