WD disintegration by accretion of companion star is not implosion, thats very clear, but is it explosion? The WDs are mainly with carbon and oxygen in the core, so infalling material of companion star ignites the carbon, that starts the burning of the carbon, the current understanding is that even though huge amount of energy (in excess of 10^30 ergs) is released in this process, but it is not explosion. In popular terms it can be considered as explosion. Type II (not 11) S/N process is far more difficult to understand and models are original star mass dependent. A star of original mass of around 5 - 25 solar mass will be just short of collapsing to form a BH, so we should get S/N type event for upto a star mass of 25 solar mass. In the lower range at the end of exothermic fusion process (till Fe56), the gravitational collapse starts, this is implosion (outer pressure is more than inward), this is a very dynamic process, very very transient and produces Neutrons in the innermost core by electron capture process and the inner cores even traps Neutrinos (massive energy), the neutrons by nature are very less compressible and hence when outer collapses on to the inner hard surface of neutrons/nuclear matter, it bounces back, this creates the shockwave in outward direction, and if this shockwave is strong enough to destroy the secondary Fe layer, then tremendous amount of energy gets released and the star is completely destroyed sans the inner neutron star core, this is explosion, massive explosion. So it can be safely said that type II S/N is a process which starts with implosion and ends with explosion. So I would say Type I is certainly not implosion, it is explsoion in popular sense but it can be termed as burning which is just short of explosion. Type II is started with implosion (inward collapse) and then massive explosion due to shock wave.