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.
You now gain 40 more Damage Resistance and do 40% more damage while Nerd Rage is in effect. Kills you make while enraged restore some lost Health.
Note that unlike previous versions of Nerd Rage, due to the diminishing returns of Damage Resistance in Fallout 4, one should no longer consider the primary benefit of Nerd Rage to be the additional damage reduction, although it does help in a pinch. Instead, the slow-time effect is the primary defensive (and offensive) mechanism, lasting about as long as a normal (unmodified by Chemist) dose of Jet.
The effect does not last as long as you are below 20%, in a break with previous versions of the perk.
It is possible to repeatedly re-trigger the effect by healing yourself above 20% and then taking enough damage to drop back down below 20% health.
If time is already slowed from a Jet-based drug, then you won't get much extra benefit from the slowed time, though you still gain the damage and damage resistance bonuses.
There can be buggy interactions with if this perk triggers during VATS (giving you the bonus damage and resistance but not the slow-time effect), though re-entering and leaving VATS tends to help.
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.
pc In Fallout 4 Nerd Rage may stop activating when health drops below 20%. This issue can be temporarily remedied by bringing up the Pip-boy then closing it while your health is under 20%. It is not currently known what causes the issue.