  To better constrain the emission mechanism underlying the hard state of
galactic black hole candidates, we use high-time resolution RXTE lightcurves
for Cyg X-1 and GX 399-4 to compute two higher order variability statistics for
these objects, the skewness and the Fourier bispectrum. Similar analyses, in
particular using the skewness measure, have been attempted before, but the
photon collection area of RXTE allows us to present results of much greater
statistical significance. The results for the two objects are qualitatively
similar, reinforcing the idea that the same basic mechanisms are at work in
both. We find a significantly positive skewness for variability timescales less
than $\sim 1 $ second, and a {\it negative} skewness for timescale $1- 5 $ sec.
Such a skewness pattern cannot be reproduced by the simplest shot variability
models where individual shots have a fixed profile and intensity and are
uncorrelated in time. Further evidence against simple shot models comes from
the significant detection of a non-zero bicoherence for Fourier periods $\sim
0.1-10$ sec, implying that significant coupling does exist between variations
on these timescales. We discuss how current popular models for variability in
black hole systems can be modified to match these observations. Using simulated
light curves, we suggest that the most likely way to reproduce this observed
behavior is to have the variability come in groups of many shots, with the
number of shots per unit time fitting an envelope function which has a rapid
rise and slow decay, while the individual shots have a slow rise and a rapid
decay. Invoking a finite energy reservoir that is depleted by each shot is a
natural way of producing the required shot correlations.
