FM low freq w/ high?

I recall reading that it is possible to frequency modulate a low frequency with a higher one, as opposed to the usual other way around.

For example, FM a 100Hz sinewave modulated by a 1KHz sinewave.

Does anyone have any idea how this would work in practice?

I generated this in CoolEdit (audio waveform editor) and it does not look like a normal FM signal, of course. The high frequency just rides upon the lower one as a superimposed AM.

Is that all there is to it?

Robert