Experiments! I've been trying out mechanisms that use pneumatic power.
In this mechanism the top block with the air hole is a valve, below that are two opposing pistons.
The pistons move the valve, the valves move the pistons and so the whole thing goes back and forth – or that's the theory. Unfortunately is doesn't work. It is really close though. I reckon I need to make the air holes bigger, to let more air flow through.
So, I'll make some modifications and try another iteration.