I am playing a Super Strength/Will Power Brute
When attacking an NPC, instead of taking the normal amount of damage from an attack, they will take a severely reduced amount, such as 2pts instead of 50+pts. The NPC will display knock up, and effects from the hit, but where knock out blow will usually take most of a minions health, it will instead be so low as to not register. The NPC will react this way to subsequent attacks for a given time period, around 10-15 seconds or so and will then eventually revert to taking normal damage.
This has occurred for council, lost, circle of thorns, clock work and sky whatevers. It is random, and does not happen to all npcs, I would say it might be around a 5-10% chance to occur.