The ball was filled with black powder, a fuse was in the ball and was usually lit when the cannon fired(by the firing of the cannon). Some mortars had their fuse lit before the round was fired. They were more or less timed to go off at the range they were being fired at, I believe they were adjustable for time. This is a rough explanation and more detailed info can be obtained by your friendly search engine.
Stuff the potato down the neck with a broom handle, add a little starter spray to the back chamber (2-1/2" ABS), quickly screw the voss plug in, and hit the sparker. It was a good way to get rid of potatoes that were getting too soft to cook.