  Many explosive astrophysical events, like magnetars' bursts and flares, are
magnetically driven. We consider dynamics of such magnetic explosions -
relativistic expansion of highly magnetized and highly magnetically
over-pressurized clouds. The corresponding dynamics is qualitatively different
from fluid explosions due to the topological constraint of the conservation of
the magnetic flux. Using analytical, relativistic MHD as well as force-free
calculations, we find that the creation of a relativistically expanding,
causally disconnected flow obeys a threshold condition: it requires
sufficiently high initial over-pressure and sufficiently quick decrease of the
pressure in the external medium (the pre-explosion wind). In the subcritical
case the magnetic cloud just "puffs-up" and quietly expands with the pre-flare
wind. We also find a compact analytical solution to the Prendergast's problem -
expansion of force-free plasma into vacuum.
