The emitter can be controlled although it is limited. Formulas can be entered in strength and rate but not lifetime. As far as the author of this article knows, the emitter only supports the values of on, (1) and off, (0). Any value over 0 will turn the emitter on. A value below 0 such as -1 will cause the emitter to fire the opposite direction. This means your emitters are limited in control.