Precise timing is required for this trick. Build a Firestorm generator. 
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 be fooled 
into thinking that you still have the generator. You can turn it off by 
building another generator and re-clicking on the Firestorm button.