Hidden blade prop from Assassin's Creed games. This has a dual-action switchblade mechanism that allows it to extend and retract triggered by a ring on the user's finger.
At least in theory. I was never able to tune the mechanism so that it would both extend and reliably retract. I'm giving it up for now, but the design is free to modify in case someone else is able to perfect it.
The video shows the assembly sequence. Here are a few notes on the steps:
The shell should be smooth and flat on the inside
The blade needs to slide freely in the shell
The latches are placed to lock the blade in the retracted and extended positions. They should bend out of the way as the blade moves and then snap into place
The kicks need to slide freely in their slot, and are under tension with a rubber band between them
With the slide in place holding the kicks apart, the device will start to work. Move the slide forward to extend the blade, backwards to retract
Rotation of the cam moves the slide forward and back
The triggering line needs to be strong and thin. I used 26-gauge stranded wire. There are two bumps in the line, one to catch in the cam and another to stop the line from retracting too far. I used small bits of metal tubing clamped to the wire. The line needs to be fixed at the back end with a small rubber band or spring to make it retract after use. The other end is attached to a ring on the user's finger