The birth of a neutron star with an extremely strong magnetic field, called a magnetar, has emerged as a promising scenario to power a variety of outstanding explosive events. This includes gamma-ray bursts, supernovae with extreme kinetic energies called hypernovae and super-luminous supernovae. The origin of these extreme magnetic fields (of the order of 10^15 Gauss) remains, however, obscure and requires an amplification over several orders of magnitude during the formation of the neutron star. I will describe our current understanding of the physical processes that may lead to this magnetic field amplification and first results on the dependence of the explosion properties on the magnetic field geometry.