Unlike a DoT's locked-in buff state, Fortify's damage-reduction effect is recalculated fresh for every hit you take, comparing your CURRENT Life to your CURRENT Fortify amount at the moment of that specific hit.
Fortify grants 10% damage reduction, but only for the portion of an incoming hit that would otherwise drop your Life to at or below your current Fortify value - not for the whole hit, and not based on any past state. If your Life is already above your Fortify amount, the top portion of a big hit gets none of this reduction; only the portion that would push you into/through your Fortify band benefits.
This is a live, per-hit overflow calculation rather than a stored buff: the game checks your Life and Fortify values at the exact moment damage is being applied for every incoming hit, with no memory of past Fortify totals.
Worked community example: 1,000 Life, 500 Fortify, hit for 600 damage. You end at 410 Life. The first 500 points of the hit (dropping you from 1000 to 500) get NO reduction; the remaining 100 damage (which would push you below your 500 Fortify) gets the 10% reduction, becoming 90 - hence ending at 1000-500-90=410 rather than 1000-600=400.
Fortify protects you best when your current Life is close to or below your Fortify amount already (e.g. after taking some chip damage); topping off to full Life right after a big Fortify application can waste most of the mitigation on your next hit, since the top part of your Life bar sits above the Fortify band.
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