  IGR J17091-3624, in addition to GRS 1915+105, is the only black hole X-ray
binary that displays ``heartbeat"-like variability characterized by structured
flares with high amplitudes. In this study, we conduct a detailed
phase-resolved analysis of the recently identified ``heartbeat"-like Class X
variability in IGR J17091-3624 during its 2022 outburst, utilizing data from
NICER and NuSTAR observations. A shortage in the high-energy (>20 keV) X-ray
flux is detected at peak phases of the soft X-ray flare at a ~15 sigma
confidence level from the phase-folded light curves. Furthermore, our
phase-resolved spectral analysis reveals variations in the spectral shape,
particularly showing significant synchronous variations in the disk temperature
and flux with the count rate. These findings imply that the flare is primarily
driven by instabilities within the accretion disk, consistent with previous
studies on the well-known Class rho variability in GRS 1915+105. However, we
also observe a positive correlation between the disk temperature and flux over
the flare cycle, which differs from a loop relation between the two parameters
found in the Class rho variability. This could suggest differences in
underlying physical processes between the two variability classes. Variations
in the Componization component during flares are also observed: the electron
temperature and covering fraction show anti-correlations with the disk flux,
revealing potential interactions between the accretion disk and the corona
during these flares.
