Physical damage dealt to a shield is fully applied and does not store damage.
Each tick of damage received is stored and dealt individually, regardless on when other hits were received.
The healing value is stored on-takedowns, working similarly to the damage storage. Meaning: Getting a takedown while the healing is already going on, the duration will be refreshed to 2 seconds and the new heal value will be added to the remaining.
The unique passive's usage is best realized when looking at a simplified situation where an enemy inflicts instant burst damage onto you.
As a result, your opponent has to overkill you to actually finish you off within this time span, which slightly increases your time to react and deploy counter measures like shields or healing. At the same time it also slightly improves your time window for retaliation, and is as such most powerful in a fight between two assassins.
Regardless, it's important to not overestimate this unique passive's influence, as most assassins generally overkill an opponent, and as such aren't hindered by slightly increased health.
Unique Passive - Ignore Pain: Stores ( 35% / 15%) of all post-mitigationphysical damage received, which is successively taken as damage over time true damage instead, dealing one-third of the stored damage every second.
Unique Passive - Defy: Champion takedowns grant you 30% bonus movement speed for 2 seconds, remove Ignore Pain's remaining damage, and heal you for 10% of your maximum health over the duration.
Unique Passive: Stores ( 30% / 10%) of allpost-mitigationphysical and magic damage received, which is successively taken as damage over time true damage instead, dealing one-third of the stored damage every second.