Sorry for the ignorance, but what is a Sustainiac Guitar?
It's a reference to the Sustainiac system, identical in all but name to the Fernandes Sustainer system. Short story: induces feedback on a particular note you choose so that you either get infinite sustain, or harmonic feedback above the note infinitely.
Guitar pickups work by measuring the change in magnetic field density (flux), which is why your guitar strings are made from nickel, iron and/or cobalt (ferrous, magnetic metals). As the strings move they induce a voltage in the pickup which is then amplified using your (hurr durr) amplifier.
A sustainer works backwards, it induces a change of magnetic field density using a magnet, which can induce vibration in the guitar string. It uses a signal from another pickup (typically the bridge pickup) so it knows what pitch it should create. The harmonic modes just mean it multiplies this by some number to get a particular octave.
Quick and easy explanation. If you'd like a better explanation or have any questions please ask.