OK JUST as an example... (I lived in Europe... I do dangerous things...) NOT FOR ACTUAL USE but for illustration purposes... if you has two identical Metal hole saws...
http://media.incomsupply.com/products/338672.gif
and Meshed them teeth to teeth, on a shaft, they would lock in one direction. One attached to the winding spool (fixed in place) , one attached to the motor such that it can retract away from the ... winding spool hole saw... , You can wind the spring back... Retract the Motored saw... and Yell "SCOTLAND Es FRE'!!!! Or something equally appropriate when looking for deadly outcome with a projectile...
The concept is having two plates that lock and then releasing them, by separation.. re engaging them to start the cycle over...
Um.. In case you missed it... DON'T ACTUALLY USE the Metal Hole Saws!
In fact... forget I said anything!
Steve