  We describe a mechanism by which a failed deflagration of a
Chandrasekhar-mass carbon-oxygen white dwarf can turn into a successful
thermonuclear supernova explosion, without invoking an ad hoc high-density
deflagration-detonation transition. Following a pulsating phase, an accretion
shock develops above a core of 1 M_sun composed of carbon and oxygen, inducing
a converging detonation. A three-dimensional simulation of the explosion
produced a kinetic energy of 1.05E51 ergs and 0.70 M_sun of 56Ni, ejecting
scarcely 0.01 M_sun of C-O moving at low velocities. The mechanism works under
quite general conditions and is flexible enough to account for the diversity of
normal Type Ia supernovae. In given conditions the detonation might not occur,
which would reflect in peculiar signatures in the gamma and UV-wavelengths
