  One of the youngest known remnants of a core-collapse supernova (SN) in our
Galaxy is G320.4$-$1.2/MSH 15-52 containing an energetic pulsar with a very
short (1700 yr) spindown age and likely produced by a stripped-envelope SN Ibc.
Bright X-ray and radio emission north of the pulsar overlaps with an H$\alpha$
nebula RCW 89. The bright X-rays there have a highly unusual and quite puzzling
morphology, consisting of both very compact thermally emitting knots and much
more diffuse emission of nonthermal origin. We report new X-ray observations of
RCW 89 in 2017 and 2018 with Chandra that allowed us to measure the motions of
many knots and filaments on decade-long time baselines. We identify a fast
blast wave with a velocity of $(4000 \pm 500)d_{5.2}$ km/s ($d_{5.2}$ is the
distance in units of 5.2 kpc) with a purely nonthermal spectrum, and without
any radio counterpart. Many compact X-ray emission knots are moving vary fast,
with velocities as high as 5000 km/s, predominantly radially away from the
pulsar. Their spectra show that they are Ne- and Mg-rich heavy-element SN
ejecta. They have been significantly decelerated upon their recent impact with
the dense ambient medium north of the pulsar. We see fast evolution in
brightness and morphology of knots in just a few years. Ejecta knots in RCW 89
resemble those seen in Cas A at optical wavelengths in terms of their initial
velocities and densities. They might have the same origin, still not understood
but presumably related to stripped-envelope SN explosions themselves.
