  The fast variability observed in the X-ray emission from black-hole binaries
has a very complex phenomenology, but offers the possibility to investigate
directly the properties of the inner accretion flow. In particular, type-B
oscillations in the 2-8 Hz range, observed in the Soft-Intermediate state, have
been associated to the emission from a relativistic jet. We present the results
of the timing and spectral analysis of a set of observations of the bright
transient MAXI J1348-630 made with the NICER telescope. The observations are in
the brightest part of the outburst and all feature a strong type-B QPO at ~4.5
Hz. We compute the energy dependence of the fractional rms and the phase lags
at the QPO frequency, obtaining high signal-to-noise data and sampling for the
first time at energies below 2 keV. The fractional rms decreases from more than
10% at 9 keV to 0.6% at 1.5 keV, and is constant below that energy. Taking the
2-3 keV band as reference, photons at all energies show a hard lag, increasing
with the distance from the reference band. The behaviour below 2 keV has never
been observed before, due to the higher energy bandpass of previous timing
instruments. The energy spectrum can be fitted with a standard model for this
state, consisting of a thin disc component and a harder power law, plus an
emission line between 6 and 7 keV. We discuss the results, concentrating on the
phase lags, and show that they can be interpreted within a Comptonization
model.
