Many synthesizers can use alternate tunings, but you have to pick a specific tuning and stick with it. Moselle's math lets you do a huge number of things though. Here, we use a general-purpose MIDI control, General1, to fade BETWEEN the common "equal temperament" scale and "just intonation." Various common C chords are played in equal temperament, then the tuning is faded to just intonation so you can hear that. (It sounds very in-tune, with no "chorusing" from being slightly detuned from mathematical perfection). Note even complex and strange note clusters sound well-tuned in just intonation.
If you look at the program closely, you can see the formula for the portamento input: Fade (General1, Tuning, TunJust7LimC). This Fade () function doesn't have anything to do with tuning specifically. It can fade between any two other things. However Moselle lets you use the math anwhere you want to do anything math can do, and this demonstrates the flexibility.
(An interesting way to use this ability would be to modulate keys--normally impossible in just intonation--by fading from a Cmaj just intonation to an Fmaj just intonation or whatever you need. Equal Temperament was invented as a compromise with just ONE purpose: to let you modulate keys. And with Moselle you could go back to just intonation if you wanted and simply fade as needed between tunings.)
Note the sound is recorded directly from Moselle: no hardware or external processing was used.