The TAO in its simple form is an additive oscillator, but one with 28 harmonics. First dial in how many harmonics you want to use, then drag the slider to set the volume for a specific harmonic.
The real beauty of the TAO however shows when you use multiple TAOs. In that case you can use each of them as either CARRIER or MODULATOR. To do that, take the output of a second (MODULATOR) TAO in to the PM or FM inputs of the first (CARRIER) TAO. This way, when dialing up the PM, you end up with sounds that remind of the DX7 or Synclavier, where you could think of each TAO as an operator. By using multiple TAOs, you can recreate the 'algorithms' of the DX7 for instance, while still having more options because every 'operator' is also an additive synthesizer.
In Chinese philosophy TAO is 'The Way' Find yours using TAO!
The manual for the module can be found here: https://requestformusic.com/RFM-VM/RFM%20TAO%20manual.pdf
An instruction video can be found here: https://www.youtube.com/watch?v=pMYerHmG6bw