Whirlpool hopping adds hops after flameout, while the wort is still hot — commonly 170-190°F (77-88°C) — during the whirlpool rest brewers use to let hot- and cold-break material settle before chilling. The residual heat still extracts real aroma and flavor compounds, but with meaningfully less isomerization (and therefore less bitterness) than the same hops added earlier during an active boil.
Sitting between kettle hopping and dry hopping
Whirlpool additions land in a genuine middle ground on the hop-timing spectrum: more bitterness contribution than a true dry-hop addition (since real heat is still present), but considerably less than a standard late-boil addition at 5-15 minutes, while extracting noticeably more aroma character than a hop that spent 60 minutes actively boiling ever could. This is a large part of the modern hazy-IPA toolkit for building intense hop aroma without pushing measured IBU as high as an equivalent amount of hops added during an active boil would.
Temperature and hold time both change the outcome
A longer whirlpool rest at higher temperature extracts more of everything — aroma oils and residual bitterness alike — while a shorter, cooler rest leans harder toward aroma with minimal bitterness contribution; brewers dial both variables deliberately depending on how much of the hop character they want the whirlpool step, rather than the boil or a later dry-hop addition, to be responsible for.