[daltore]

Depending on how you're winding up your kicking mechanism, you could use a cam. Basically, a cam is shaped like a snail shell looking at it sideways. You wind it around so the smallest radius part of the cam contacts a shaft (known as the cam follower), and by rotating the cam, it pushes on the shaft more and more. Then, when it reaches the point of highest radius, it suddenly drops off, releasing the kicker.

The advantages of this kind of design is that it's very simple to build, and *relatively* hard to break, while the main caveat is that you pretty much only have one kicking power setting.