  XSS J12270-4859 (henceforth J12270) is the first low-mass X-ray binary to
exhibit a transition, taking place at the end of 2012, from an X-ray active
state to a radio pulsar state. The X-ray emission based on archival XMM-Newton
observations is highly variable, showing rapid variations (10 s) from a high
X-ray luminosity mode to a low mode and back. A flaring mode has also been
observed. X-ray pulsations have been detected during the high mode only. In
this work we present two possible interpretations for the rapid swings between
the high and low modes. In the first scenario, this phenomenon can be explained
by a rapid oscillation between a propeller state and a radio-ejection pulsar
state, during which the pulsar wind prevents matter from falling onto the
neutron star surface. In the second scenario, a radio pulsar is always active,
the intra-binary shock is located just outside the light cylinder in the high
mode, while it expands during the low mode. At variance with other transitional
pulsars, J12270 shows two instances of the low mode: a low-soft and low-hard
mode. Performing an X-ray spectral analysis, we show that the harder component,
present in the low-hard spectra, is probably related to the tail of the flare
emission. This supports the understanding that the flare mechanism is
independent of the high-to-low mode transitions.
