  The variability of the Fe K alpha line near 6.5 keV seems to be reduced
compared to the variability of the hard X-rays which presumably drive the line
emission. This is observed both in active galactic nuclei and galactic black
hole binaries. We point out that such reduced variability, as well as lack of
coherence between the variations of the line and the continuum, are a natural
prediction of a propagation model of variability in the geometry of inner hot
accretion flow. We compute detail model predictions of the variability
characteristics which could be compared with current and future data. We also
point out that the model requires a gradual disappearance of the cold disc,
rather than a sharp transition from the cold disc to a hot flow.
