  We present radiation-hydrodynamics simulations of core-collapse supernova
(SN) explosions, artificially generated by driving a piston at the base of the
envelope of a rotating or non-rotating red-supergiant progenitor star. We
search for trends in ejecta kinematics in the resulting Type II-Plateau (II-P)
SN, exploring dependencies with explosion energy and pre-SN stellar-evolution
model. We recover the trivial result that larger explosion energies yield
larger ejecta velocities in a given progenitor. However, we emphasise that for
a given explosion energy, the increasing helium-core mass with main-sequence
mass of such Type II-P SN progenitors leads to ejection of core-embedded
oxygen-rich material at larger velocities. We find that the photospheric
velocity at 15d after shock breakout is a good and simple indicator of the
explosion energy in our selected set of pre-SN models. This measurement,
combined with the width of the nebular-phase OI6303-6363A line, can be used to
place an upper-limit on the progenitor main-sequence mass. Using the results
from our simulations, we find that the current, but remarkably scant, late-time
spectra of Type II-P SNe support progenitor main-sequence masses inferior to
~20Msun and thus, corroborate the inferences based on the direct, but
difficult, progenitor identification in pre-explosion images. The narrow width
of OI6303-6363A in Type II-P SNe with nebular spectra does not support
high-mass progenitors in the range 25-30Msun. Combined with quantitative
spectroscopic modelling, such diagnostics offer a means to constrain the
main-sequence mass of the progenitor, the mass fraction of the core ejected,
and thus, the mass of the compact remnant formed.
