  M1-92 can be considered an archetype of bipolar pre-planetary nebulae. It
shows a clear axial symmetry, along with the kinematics and momentum excess
characteristic of this class of envelopes around post-AGB stars. By taking
advantage of the new extended configuration of the IRAM Plateau de Bure
interferometer, we wanted to study the morphology and velocity field of the
molecular gas better in this nebula, particularly in its central part. We
performed sub-arcsecond resolution interferometric observations of the J=2-1
rotational line 13CO M1-92. We found that the equatorial component is a thin
flat disk, which expands radially with a velocity proportional to the distance
to the center. The kinetic age of this equatorial flow is very similar to that
of the two lobes. The small widths and velocity dispersion in the gas forming
the lobe walls confirm that the acceleration responsible for the nebular shape
could not last more than 100-120 yr. The present kinematics of the molecular
gas can be explained as the result of a single brief acceleration event, after
which the nebula reached an expansion velocity field with axial symmetry. In
view of the similarity to other objects, we speculate on the possibility that
the whole nebula was formed as a result of a magneto-rotational explosion in a
common-envelope system.
