Mechanics of Art of Combat Skills (Hidden Skills)

Art of Combat Skills (Hidden Skills): Behave the same as other Smithing Texts in terms of drop rate and drop mechanics. Rather than unlocking armor or weapon pieces they unlock Hidden Skills for the Weapons, Ninjutsu, Onmyo Magic, and Shiftling trees. All Art of Combat Skills will drop as an Exotic Item regardless of the difficulty. Once accquired that skill will automatically unlock in that specific Skill Tree.

How Luck affects Hidden Skills: Luck does NOT affect the drop rate of Smithing Texts and Hidden Skills. Luck only affects the rarity of items that drop, NOT the chance of an item dropping. Buffs like Luck Bringer Talisman, and Soul Purge Talisman do not affect the Smithing Text and Hidden Skills drop rate. Tempering Luck+ attributes on armor, accessories, and Soul Cores also does not affect the Smithing Texts and Hidden Skills drop rate.

How Item Drop Rate affects Hidden Skills: Item Drop Rate buffs DOES affect the Smithing Text and Hidden Skills drop rate. There is no active buff that we can use to manually increase our IDR at will, but we can do 3 things to boost our drop rate chances.
1) Temper on the attribute "Item Drop Rate" onto all our Armor pieces, Accessories, and use Soul Cores with this attribute (if available).
2) Use the Guardian Spirit Itokuri as our primary Guardian Spirit.
3) Equip the Hunter Blessing as our Kodama Blessing, and use the Head Armor Piece: Kodama Bowl (mission reward: The Search) to boost our Kodama Blessing by 25%.

The Drop rate of Hidden Skills and Smithing Texts is the same regardless of difficulty.