Dry hopping adds hops after the boil has finished, typically during active or post-fermentation conditioning, specifically to extract aroma and flavor oils without the bitterness a boil addition would contribute — with no boiling involved, the alpha acids in a dry-hop addition largely don't isomerize into soluble bitterness the way a kettle addition does.
Why the Tinseth formula reads it as almost nothing
This is where a genuinely important quirk of IBU calculation shows up directly: because Tinseth's utilization formula reaches zero at zero boil minutes, a dry-hop addition contributes essentially nothing to a Tinseth-calculated IBU number, even when a huge quantity of hops went in. Rager's formula still credits a small amount, since its curve doesn't collapse to zero the same way — see the New England IPA style guideline for a worked example showing exactly how far apart the two formulas land on the same large dry-hop addition.
Timing and duration are real recipe variables, not afterthoughts
Dry hop contact time (commonly a few days to just over a week) and whether it happens during active fermentation or after it's finished both genuinely affect the extracted aroma character — too long a contact time risks grassy, vegetal off-notes from extended exposure to hop plant material, while too short a time under-extracts the aroma compounds a brewer is chasing in the first place.