This is something that i am quite obsessed with, figuring out how the hidden blade works. to be able to acutally make one myself. i have an idea on how to make a mechanism, witch will require a spring simply designed to retract itself, a slide, a string to pull out the blade and the locking mechnism, my idea for the lockingmechanism is simple. the one who wields the blade simply pulls the string and the blade shoots out(kind of obviuos). when the blade i extended a piece of metal suprressed by the blade itself when the blade i restracted pops up. allowing the wielder to stab any object. when the wielder want's to retract the blade a button on the back of the whole thing is pushed simply by bending the arm. this button is attachted to the piece of metal holding the blade in place. when the button is push the piece of metel is pulled down, and the blade retracts, because of the spring.