  We discuss the use of the bicoherence - a measure of the phase coupling of
oscillations at different frequencies - as a diagnostic between different
models for high frequency quasi-periodic oscillations from Galactic black hole
candidates. We show that this statistic is capable of finding qualitative
distinctions between different hot spot models which produce nearly identical
Fourier power density spectra. Finally, we show that proposed new timing
missions should detect enough counts to make real use of this statistic.
