Never seen one fired, but I gather it drops down the tube and hits a firing pin at the bottom, that activates a primer in the shell, that ignites the propellent.
What confuses me is first, if you are just hitting the tail against a flat surface, how would that activate the primer. And if the primer goes off, wouldn't that activate the propellent.
In other words, if slapping it down activated the primer, wouldn't it blast out of the persons hand when the propellent was ignited.
Must be that slapping it down somehow arms the fuse for the explosive and not the propellent.