  We show that the 216.8$\pm$0.1 ms periodicity reported for the fast radio
burst (FRB) 20191221A is very constraining for burst models. The high accuracy
of burst periodicity (better than one part in 10$^3$), and the 2\% duty cycle
(ratio of burst-duration and inter-burst interval), suggest a pulsar-like
rotating beam model for the observed activity; the radio waves are produced
along open field lines within $\sim 10^7$ cm of the neutron star surface, and
the beam periodically sweeps across the observer as the star spins. According
to this picture, FRB 20191221A is a factor $\sim 10^{12}$ scaled up version of
galactic pulsars with one major difference: whereas pulsars convert rotational
kinetic energy to EM waves, the outbursts of 20191221A require conversion of
magnetic energy to radiation.
