Onager - it really works!

Not shown in this model are the torsion bands and the arming/triggering system:

(1) Torsion Bands - technic rubber bands are fed across the framework through
the technic beam holes and looped around the technic 3 axles on either side.  
The arm simply passes in the middle of these bands.  The technic axles on either
side are then rotated a number of times to achieve the desired torsion, which can
snap the arm up and send a payload flying

(2) Arming/Trigger System - 2 strings are needed.  Each one is tied to the technic 
engine piston cam.  One of the string is then affixed to the winding mechanism
bar at the rear of the onager.  The other is left free.

To arm the onager:  the engine piston cam piece is inserted into the catch on the 
main arm.  The crank is then turned to pull the arm back as far as desired.  When
ready to fire, the free string is yanked, pulling the technic engine cam out of 
the catch and letting the arm snap up.

Model can also have wheels added, and hooked to a team of horses for use as 
mobile artillery :)

-s