/diy/ - Do It Yourself

Ok so i bought

>Platt, Make: Electronics

And i have a question in one of the circuits, pic related

He says that once one of those 4 NC contacts is open, the tension on the base of the transistor will go up, and the relay coil will be activated, so the realy stays turned on, but you need to add a diode (1N4001) to prevent reverse tension in the emitter of the transistor

But how is it possible that such reverse tension could exist? Isn't the emitter of the transistor silicon doped, and has electrict negative charges? Its a NPN by the way

Surely the resistence to go trough the emitter is higher than the coil of the relay? What am i missing