hope someone can chime in better, as i have interest in anons views who are better versed.
the resonance is that of the earth.
a ultra low frequency gentle hum that the body and cells feel but is sub audible.
i think there is a an interaction with our bodies frequencies whereby people interacting can match each others frequencies and increase amplitude enough to affect the resonaces. the body likely is affected by the schumann as a base vibration, but emotions and neurological/chemical balances of the body can make it vary, when a lot of people experience the same emotions that vary from schumann R then humans/spiritual may create enough amplitude to affect schumann R.
there are geological and weather affects that affect Schumann Rs, but i think the interest is in how humans can.
perhaps occult and witchez have ceremonies on satanic holiday and at a certain time and Schumann may detect that negative frequency so it may vary with both + and - energy.