My first attempt (used for popping balloons) was to use a simple mechanical method: I used a needle-tipped "arrow" as a ballistic pendulum to pop the balloons. On the back of the pendulum is a thread that goes to a piece of paper separating a couple of contacts that go to the flash. The length of the thread is adjusted just right so that the paper is jerked out from between the contacts when the balloon pops.

Unfortunately, it's impossible to do. Well, hard, at least. The problem is that the arrow is moving very slowly relative to the speed of a popping balloon. The balloon rips at close to the speed of sound (1100 feet per second); the arrow is going maybe 5 feet per second. So, the smallest difference in the position of the arrow translates into a huge difference as far as the balloon is concerned. I managed to get maybe 1 in 10 shots that had the balloon somewhere in mid-pop. As my five-year old, James, walked away from the second evening of watching me attempt this arrangement, he said, "It seems like everything has to be just right for it to work." From the mouths of babes.