  The black hole candidate and X-ray binary MAXI J1535-571 was discovered in
September 2017. During the decay of its discovery outburst, and before
returning to quiescence, the source underwent at least four reflaring events,
with peak luminosities of $\sim$10$^{35-36}$ erg s$^{-1}$ (d/4.1 kpc)$^2$. To
investigate the nature of these flares, we analysed a sample of NICER
observations taken with almost daily cadence. In this work we present the
detailed spectral and timing analysis of the evolution of the four reflares.
The higher sensitivity of NICER at lower energies, in comparison with other
X-ray detectors, allowed us to constrain the disc component of the spectrum at
$\sim$0.5 keV. We found that during each reflare the source appears to trace
out a q-shaped track in the hardness-intensity diagram similar to those
observed in black hole binaries during full outbursts. MAXI J1535-571 transits
between the hard state (valleys) and softer states (peaks) during these flares.
Moreover, the Comptonised component is undetected at the peak of the first
reflare, while the disc component is undetected during the valleys. Assuming
the most likely distance of 4.1 kpc, we find that the hard-to-soft transitions
take place at the lowest luminosities ever observed in a black hole transient,
while the soft-to-hard transitions occur at some of the lowest luminosities
ever reported for such systems.
