Basic attacks on enemy champions gain increasingly more life steal based on your missing health, granting 0.06% - 20% (based on Missing health) bonus life steal. The life steal against minions is 50% as effective for melee champions and 25% as effective for ranged champions, granting 0.03% - 10% (based on Missing health) and 0.02% - 5% (based on Missing health) bonus life steal respectively.
The amount of life steal gained caps-out at 80% missing health, but does not scale linearly.
10% missing health: ~1%
50% missing health: ~9%
80% missing health: 20%
(Does not work against monsters.)