From Paragon Wiki Archive
Bruising is a damage resistance debuff that was added to the game in Issue 18.
On a successful hit, any tier 1 Tanker secondary power set attack will bruise an enemy target. This bruising effect causes a resistible 20% damage resistance debuff which lasts for 10 seconds. Bruising cannot stack, even if there are multiple tankers attacking the same target; thus, it is a good idea for multiple tankers to spread attacks out on the whole spawn.