My theory is that some, mostly older dragonators, work by using a heavy weight and a pulley to launch the prongs forward.
I have included a diagram to aid in understanding. Red lines are cable, and the grey is the fort. The brown rod is the prong. The grey block on the cable is the weight. What happens when it fires is that the weight is allowed to drop down a shaft under the machine. The cable then pulls on the end of the prong, launching it forward. To reset the weight has to be lifted again to its ready position.
Working Dragonators would likely be slightly more complex, for example having a clip in the rope so the prong can be retracted immediately after firing.
To add credibility to my theory I have constructed a small model and aim to construct a larger one. The model functioned quite well. I shall Upload photos of the larger model when finished.