  We report on NuSTAR observations of transient pulsations in the neutron star
X-ray binary SMC X-1. The transition from non-pulsing to pulsing states was not
accompanied by a large change in flux. Instead, both pulsing and non-pulsing
states were observed in a single observation during the low-flux super-orbital
state. During the high-state, we measure a pulse period of $P =
0.70117(9)\,\mathrm{s}$ at $T_{ref} = 56145\,\mathrm{MJD}$. Spectral analysis
during non-pulsing and pulsing states reveals that the observations can be
consistently modeled by an absorbed power law with a phenomenological cutoff
resembling a Fermi-Dirac distribution, or by a partially obscured cutoff power
law. The shapes of the underlying continua show little variability between
epochs, while the covering fraction and column density vary between
super-orbital states. The strength of pulsations also varies, leading us to
infer that the absence and reemergence of pulsations are related to changing
obscuration, such as by a warped accretion disk. SMC X-1 is accreting near or
above its Eddington limit, reaching an unabsorbed X-ray luminosity of $L_{\rm
X}({\rm 2-10~keV}) \approx 5 \times 10^{38}\, {\rm erg}\, {\rm s}^{-1}$. This
suggests that SMC X-1 may be a useful local analog to ultraluminous X-ray
pulsars (ULXPs), which likewise exhibit strong variability in their pulsed
fractions, as well as flux variability on similar timescales. In particular,
the gradual pulse turn-on which has been observed in M82 X-2 is similar to the
behavior we observe in SMC X-1. Thus we propose that pulse fraction variability
of ULXPs may also be due to variable obscuration.
