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.”— In-game description, Fallout: New Vegas
Whenever your health drops below half, all companions in your party gain 50 damage resistance (not damage threshold). This means companions will ignore half of the base damage of any attack they receive. This combines well with high nerve to increase their survivability in hardcore mode.
Ferocious Loyalty will remain active as long as your health is below 50%, so the effect can be thought of as sustainable.
- Hit points granted by the Lifegiver perk do not count toward the Courier's total health as far as this perk is concerned. This means the perk will not activate at 50% health as described if Lifegiver is also taken, making it harder to safely sustain the effect. Rather, the perk will activate when one-half of one's total health minus 30 is reached.