They work because the air bubbles trap water between them and carry it out of the pipe. This causes a drop in water pressure inside the sponge so water runs through the sponge. This traps "gunge" in the sponge which is food for bacteria that then live in the sponge. These bugs convert urea from the fish to ammonia then nitrite then nitrate and this is where the "cycling" takes place. Therefore, you only want it bubbling slowly as it is the water between the bubbles you want not the bubbles, and it is a living system so you dont want to clean the filter unless it gets completely blocked up as the bacteria multiply to fit the gunge available and when you do clean it you only remove enough to free it up again. Some people say to clean it in aquarium water but I think it is best to just leave it as gunged up as you can while it is still working. I clean the excess off the sponge when syphoning gunge off the bottom of the tank.