Whenever your Hit Points drop to 20% or below, your Strength is raised to 10 and you gain 50 to damage resistance. This stacks with other perks, and applies regardless of clothing, which makes it still useful even if you wear heavy armor, as it can help maintain your DR even if said armor is damaged or broken.
Whenever your health drops to 20% or below, your Strength is raised to 10 and your Damage Threshold is boosted by 15. As in Fallout 3, this effect stacks with similar bonuses, and applies even if you're naked or your armor is broken; you merely need to meet the health threshold.
Notably, the Courier's base health determines the trigger for this perk. Bonus HP from other sources such as Buffout or the Thought You Died perk will not raise the threshold.
Because Fallout 3 has a DR cap of 85, most or all of the 50 DR bonus may be ignored under certain situations.
Similarly, while Fallout: New Vegas does not feature a similar cap on DT, 20% of pre-DT damage is guaranteed to go through, regardless of how high a player's DT is. As such, in certain situations an increase in DT may yield little to no benefit.
If you're having difficulty eyeballing how much 20% of your health is, this is the same amount of health you need before you begin to hear your heart beating.