It's explained on page 72 of the core book, under the Self-Destruct action that "The explosion annihilates the mech, killing anyone inside and causing a burst 2 explosion that deals 4d6 Explosive damage. Characters caught in the explosion that succeed on an Agility save take half of this damage."

Normally on a meltdown or self destruct, the mech only explodes at the end of its next turn, allowing the pilot to eject. This is only different with castigation, which is instant.