So I'm a big fan of the video game Assassin's creed, and one of Altair's sick wepons is the hidden blade. It is basicly a folded blade in a compartment that has some sort of mechanism that shots it out and locks it into place with a gesture or movement of the hand. It then returns back into the compartment with another gesture or movement of the hand. It is straped to the bottom of the forearm. Much thanks if you could do a mythbuster on this because there is a large debate on the web if somthing like this can be built.

It is a bit of a stretch but i suppose it could have been made with a spring to extend it that was released when he pushed a button with his wrist (based on the motion he makes) and retracted with a rope attached to his shoulder that could work similar to how common prosthetic limbs work today.

This would never have been made (for combat at least) because after killing someone with it the blood would get in the joints and be impossible to clean causing it to rust and seize up. That and the apparent cost of the persons middle finger.