  We present the results of 16-s time scale spectral fits for Cygnus X-1 in the
soft state and in the transition state, using a Comptonised blackbody plus an
iron line. On these timescales, we find that that the continuum source flux can
vary by factors 2-3 and that the iron line intensity appears to track these
changes well, i.e., the inferred equivalent width of the line remains constant
to within the errors. We also find no significant changes in the seed
(blackbody) photon temperature, while the properties of the Comptonising corona
clearly do vary, with the spectral hardness and flux generally being
correlated. The corona therefore seems to be the overall driver for the rapid
timescale variability observed in the soft and transition states. These results
are consistent with the Fourier resolved spectroscopy results of Gilfanov et
al. (2000) that indicate the iron line shows rapid flux variations while the
blackbody component does not and suggest that the iron line flux in fact tracks
continuum changes down to very short timescales. We extend this work by showing
that not only the variability amplitudes, but also the {\it phases} of the iron
line and continuum components are identical. We note that the short timescale
variability properties of the soft and transition states are actually not very
different from those of the hard state, suggesting that the corona is the main
cause of rapid variability in that state too, and hence that the mechanism
responsible for the corona is similar in all three states.
