  Fast Radio Bursts (FRBs) are cosmological sub-second bursts of coherent radio
emission, whose source is still unknown. To date, the galactic magnetar SGR
1935+2154 is the only astrophysical object known to emit radio bursts akin to
FRBs, albeit less powerful, supporting suggestions that FRBs originate from
magnetars. Many remarkable properties of FRBs, e.g. the dichotomy between
repeaters and one-off sources, and their power-law energy distributions (with
typical index $\sim 2-3$), are not well understood yet. Moreover, the huge
radio power released by the most active repeaters is challenging even for the
magnetic energy reservoir of magnetars. Here we assume that FRBs originate from
co-rotating hot-spots anchored in neutron star magnetospheres and get
occasionally amplified by large factors via gravitational self-lensing in the
strong NS field. We evaluate the probability of amplification and show that (i)
a power-law energy distribution of events $\propto E^{-(2- 3)}$ is generally
expected, (ii) all FRB sources may be regarded as repeating, their appearance
as one-off sources or repeaters being determined by the critical dependence of
the amplification probability on the emission geometry and source orientation
relative to Earth and (iii) the most active repeaters, in particular,
correspond to extremely rare and finely-tuned orientations ($\sim$ one in
$10^6$), leading to large probabilities of amplification which make their
bursts frequently detectable. At the same time, their power release appears
enhanced, typically by factors $\gtrsim 10$, easing their energy budget
problem.
