This is part of "Skill-use to levelup skills", i.e. exp goes to skill exp.
Example:Fire resistance (diminishing return passive skill) is leveled by being hit by fire.*Same with passives that apply on being hit.
Example2: Evade(chance to evade, diminishing return skill) is leveled by evading while being hit(initial chance 1%).*Same with chance-to-X passives.
Example3:Weapon mastery is leveled by using a specific weapon succesfully(hit).
Example4:Spell mastery is leveled by using one of spells in group (e.g. fire mastery).