To fire the gun,

1. The gun's crank is worked twice, once to load, once to cock.
2. The green handle is pulled
3. which lowers the red cam follower onto the cam wheel.
4. When the cam raises the follower, the blue rod is pushed against the spring.
5. When the pilot presses the purple firing button, inside the breech block the cable lowers the blue bridge piece
6. so that when the blue rod is activated by the cam, the yellow trigger bar is pushed
7. and the gun fires.

Based on a diagram by Harry Woodman, Aeroplane, December 2004.