Ferocious Loyalty

 is a perk in Fallout: New Vegas.

Effects
The power of your personality inspires die-hard loyalty from your followers. When you drop below 50% health, your companions temporarily gain much greater resistance to damage.

When PC health drops below 50%, all companions actually does not get any extra DT.Instead, they gain a significant amount of damage resistance. The effect could be extremely effective that makes your companions about 10 times tougher than their original condition no matter how many DT they have.