Bloodstained: Ritual of the Night/Techniques

Techniques are special attacks that Miriam can perform with a specific set of weapons and a command input. There are bookshelves in the castle that add journal entries with the details of a technique, but they are not required to unlock the technique. You only need to have one of the listed weapons equipped when you input the command. Using a technique on an enemy will progress its mastery gauge. Once it is mastered, any weapon of the same type can be used to perform the technique.

There is one additional hidden technique for the Valkyrie Sword that won't show up in the technique list in the Archives menu. If you master every technique, you can perform the hidden technique by pressing  +.