What mechanism ultimately halts the extreme compression of the iron core during implosion?

The incredibly stiff resistance of nuclear matter, after protons and electrons are squeezed into neutrons.

The collapse of the iron core continues unabated until the density reaches an almost unimaginable level, where the physical constraints of matter itself come into play. During this final compression, protons and electrons are forced together via inverse beta decay to form neutrons. The collapse stops only when the core material achieves such high density that it exhibits an 'incredibly stiff resistance,' a property inherent to nuclear matter resisting further compression beyond its equilibrium state. This resistance acts as a physical wall, causing the imploding material to rebound and generate the shockwave necessary to trigger the supernova explosion.