Negates the part of boosts that lower stats by a difinitive number (NOT a percentage!). Diligence won't stop familiars or items being applied that reduce stats, and won't effect corruption. Diligence will also not effect boots from opponants that effect your stats, such as if you incur waning or bleeding.
Diligence will also not negate the following boosts: Weak Boosts, Negate Boosts, Greed, Pride, Gluttony, Instant Famine, You Shall Not Pass.
Diligence also negates Sloth.