  X-ray low frequency quasi-periodic oscillations (LFQPOs) of black hole X-ray
binaries, especially those type-C LFQPOs, are representative timing signals of
black hole low/hard state and intermediate state, which has been suspected as
to originate due to Lense-Thirring precession of the accretion flow. Here we
report an analysis of one of the \emph{Insight}-HXMT observations of the black
hole transient MAXI J1820$+$070 taken near the flux peak of its hard spectral
state during which strong type-C LFQPOs were detected in all three instruments
up to photon energies above 150 keV. We obtained and analyzed the
short-timescale X-ray spectra corresponding to high- and low-intensity phases
of the observed LFQPO waveform with a spectral model composed of Comptonization
and disk reflection components. We found that the normalization of the spectral
model is the primary parameter that varied between the low and high-intensity
phases. The variation in the LFQPO flux at the hard X-ray band (> 100 keV) is
from the Compton component alone, while the energy-dependent variation in the
LFQPO flux at lower energies (< 30 keV) is mainly caused by the reflection
component with a large reflection fraction in response to the incident Compton
component. The observed X-ray LFQPOs thus should be understood as manifesting
the original timing signals or beats in the hard Compton component, which gives
rise to additional variability in softer energies due to disk reflection.
