Mini Catapult - Trigger mechanism

I printed this cool mini catapult by ATECH:
http://www.thingiverse.com/thing:438870

Then I designed some supports for the base, in order for it to be less flexible, and also so that the catapult could rest flat on the surface despite the rubber bands going under it:
http://www.thingiverse.com/thing:865607

The last thing I wanted to add was some sort of triggering mechanism, and so I designed this, taking advantage of one of the rubber bands used for shooting to also hold it into place.