Originally Posted by
Xenith
Those with non existent or little psychic power were turned into Rubric marines. those with psychic talent found their powers amplified. That's the way it has always been.
The armour acts as a trap for their physical remains, the dust, as well as their souls. If the armour is breached, the soul can escape to the warp.
Newer fluff, but I don't have a source, seems to indicate that a TS Sorceror can recall a soul that has fled the armour by re-sealing the armour, making the only way to truly kill a rubric marine is to obliterate the armour.