Perfect timing is required for this trick. Build a Firestorm generator and when it is ready, click to activate it and blow it up as soon as it becomes activated. This works best if you have a Ghost Stalker. Have the Ghost Stalker place C4 on the generator, then when it flashes, quickly turn it on and it will automatically explode. The game will bthink that you still have the generator. You can turn it off by building another generator and re-clicking on the Firestorm button.