This gem adds a more Melee Physical Damage modifier to attacks made with the supported skill, but only when you are on full life. Being even one life below your maximum will nullify the effect.
Modifiers that use the word more act as multipliers, and stack multiplicatively with all other modifiers, while increased modifiers will stack additively with other increased modifiers.
This gem will only affect the physical damage portion of any melee attacks made with the supported skill.