Taken from the League of Legends wiki:
UNIQUE: Restores 20 health over 10 seconds after taking damage from an enemy champion.
Let's say I'm playing some marksman in the bottom lane, and my support is Janna. If Janna shields me, and then the enemy marksman autos me once but doesn't destroy the Janna shield, will I still get the +20 health effect?
The reason I ask is because I have seen a handful of very rare occasions where (for example) champion A has a shield and tower dives, taking a bunch of turret shots, and then champion B attacks them once but is unable to pierce the shield. If the only damage done by champion B was to champion A's shield, champion A will be executed by the turret instead of killed by champion B.