
Harmonics are present in any form of singing but, in harmonic singing, the singer creates stronger resonances around selected harmonics by shaping the throat very differently than does a classic singer. This particular singing technique highlights frequencies that are already present in the voice, but would remain unnoticed without the extra emphasis.
This emphasis serves as a metaphor for the process of healing. You may be getting better, but not yet be aware of it. Sometimes, it only takes listening to a sound to flip a switch in the mind. This is the intrinsic nature of sound healing.