It uses the simple mechanism of compressing spheres with tracers attached to them and when s is pressed, it toggles the action for the box to delete, therefore making an "explosion" with the tracers attached to spheres making it look like an exposion. Simple and common. I have made a save of a volcano exploding that uses the same reaction withouth the toggling of deleting of anything.
I know, I was dieing to post it like a couple of years ago, but couldn't cuz forgot my account password D: on the other hand, it's one of the fastest things in Algodoo!