  Although low-frequency quasiperiodic oscillations (LFQPOs) are commonly
detected in the X-ray light curves of accreting black hole X-ray binaries,
their origin still remains elusive. In this study, we conduct phase-resolved
spectroscopy in a broad energy band for LFQPOs in MAXI J1820+070 during its
2018 outburst, utilizing Insight-HXMT observations. By employing the
Hilbert-Huang transform method, we extract the intrinsic quasiperiodic
oscillation (QPO) variability, and obtain the corresponding instantaneous
amplitude, phase, and frequency functions for each data point. With
well-defined phases, we construct QPO waveforms and phase-resolved spectra. By
comparing the phase-folded waveform with that obtained from the Fourier method,
we find that phase folding on the phase of the QPO fundamental frequency leads
to a slight reduction in the contribution of the harmonic component. This
suggests that the phase difference between QPO harmonics exhibits time
variability. Phase-resolved spectral analysis reveals strong concurrent
modulations of the spectral index and flux across the bright hard state. The
modulation of the spectral index could potentially be explained by both the
corona and jet precession models, with the latter requiring efficient
acceleration within the jet. Furthermore, significant modulations in the
reflection fraction are detected exclusively during the later stages of the
bright hard state. These findings provide support for the geometric origin of
LFQPOs and offer valuable insights into the evolution of the accretion geometry
during the outburst in MAXI J1820+070.
