  GRB 120422A is a low-luminosity Gamma-ray burst (GRB) associated with a
bright supernova, which distinguishes itself by its relatively short T90 ~ 5 s
and an energetic X-ray tail. We analyze the Swift BAT and XRT data and discuss
the physical implications. We show that the early steep decline in the X-ray
light curve can be interpreted as the curvature tail of a late emission episode
around 58-86 s, with a curved instantaneous spectrum at the end of the emission
episode. Together with the main activity in the first ~ 20 s and the weak
emission from 40 s to 60 s, the prompt emission is variable, which points
towards a central engine origin, in contrast to the shock breakout origin as
invoked to interpret some other nearby low-luminosity supernova GRBs. The
curvature effect interpretation and interpreting the early shallow decay as the
coasting external forward shock emission in a wind medium both give a
constraint on the bulk Lorentz factor \Gamma to be around several. Comparing
the properties of GRB 120422A and other supernova GRBs, we found that the main
criterion to distinguish engine-driven GRBs from the shock breakout GRBs is the
time-averaged luminosity, with a separation line of about ~ 10^48 erg s^-1.
