  The geometry of the inner accretion flow in the hard and hard-intermediate
states of X-ray binaries remains controversial. Using NICER observations of the
black hole X-ray binary MAXI J1820+070 during the rising phase of its 2018
outburst, we study the evolution of the timing properties, in particular the
characteristic variability frequencies of the prominent iron K$\alpha$ line.
Using frequency-resolved spectroscopy, we find that reflection occurs at large
distances from the Comptonizing region in the bright hard state. During the
hard- to soft transition, the variability properties suggest the reflector
moves closer to the X-ray source. In parallel, the peak of the iron line shifts
from 6.5 to ~7 keV, becoming consistent with that expected of from a highly
inclined disc extending close to the black hole. We additionally find
significant changes in the dependence of the root-mean-square (rms) variability
on both energy and Fourier frequency as the source softens. The evolution of
the rms-energy dependence, the line profile, and the timing properties of the
iron line as traced by the frequency-resolved spectroscopy all support the
picture of a truncated disc/inner flow geometry.

High Energy Astrophysical Phenomena