ha ha, thanks!!
No the knife extends for as long as I stretch my hand. When I make a fist the knife jumps back in.
This is actually... simple to make and it does the trick for the audience... It's like tapping "up" in the game... the knife shoots out and is almost immediatly withdrawn. It just works. I might figure a way out with a lock pin that keeps it extracted but... well the reactions to the way it popped out now were quite litterly open jaws of awesomeness... I was amazed byt hat actually....
My throwing knives are not withdrawable at all. I didn' t want them to be, in order to prevent trouble with law enforcement.
The sword and dagger are withdrawable, though held firmly in place.