The MRB mentions in the Carrien section that Mutant Carrien can control their pain using Massacre Techniques. However, the book does not state what these are. I've always assumed that the Massacre Techniques are a method unique to the Mutant Carrien to control their pain. In effect they become blood-lusted during a fight, and this 'high' blocks out their otherwise ceaseless pain. This blood lust then takes a few hours to dissipate after any massacre, allowing them some respite from their pain.

Each round spent in life threatening combat gives thirty minutes that the Mutant Carrien does not feel pain for. During this time all of their combat skills are increased by two points as the pain is not affecting them. The Mutant Carrien is also immune to the affects of Fear. However, their blood lust causes them to act irrationally, forcing a compulsion to attack at any opportunity. If threatened the Mutant Carrien will have to roll vs CONC to resist charging in to combat. This roll is their last chance to resist combat that could be fatal, for example against heavily armed Operatives, or rival monsters.

The Compulsion that the Mutant Carrien feels is treated as a Psychosis, increasing in rank whenever a Mutant Carrien has survived a particularly bloody and brutal fight. This Compulsion acts as a negative to their CONC rolls, reflecting the way that older Mutant Carrien are more likely to enter combat at any opportunity.