  While the standard X-ray variability of black hole X-ray binaries (BHXBs) is
stochastic and noisy, there are two known BHXBs that exhibit exotic
`heartbeat'-like variability in their light curves: GRS 1915+105 and IGR
J17091-3624. In 2022, IGR J17091-3624 went into outburst for the first time in
the NICER/NuSTAR era. These exquisite data allow us to simultaneously track the
exotic variability and the corresponding spectral features with unprecedented
detail. We find that as in typical BHXBs, the outburst began in the hard state,
then the intermediate state, but then transitioned to an exotic soft state
where we identify two types of heartbeat-like variability (Class V and a new
Class X). The flux-energy spectra show a broad iron emission line due to
relativistic reflection when there is no exotic variability, and absorption
features from highly ionized iron when the source exhibits exotic variability.
Whether absorption lines from highly ionized iron are detected in IGR
J17091-3624 is not determined by the spectral state alone, but rather is
determined by the presence of exotic variability; in a soft spectral state,
absorption lines are only detected along with exotic variability. Our finding
indicates that IGR J17091-3624 can be seen as a bridge between the most
peculiar BHXB GRS 1915+105 and `normal' BHXBs because it alternates between the
conventional and exotic behavior of BHXBs. We discuss the physical nature of
the absorbing material and exotic variability in light of this new legacy
dataset.
