A lockable ring. It is printed as one piece, no assembly required. This is how it works, you put it on the finger of your "victim" or yourself, and then you twist the two parts and the parts inside the model will close and be locked.
To open it, simply press down on all four sides (the small 'buttons' there) and you should be able to open it.
NOTE: This model has not been printed yet and can't be guaranteed to work. Involves moving parts.