How does T-Mobile $0 down plan work?

So I am looking for a new phone, I have an iphone 4S now and I want the HTC One. I seen on T-mobiles webpage that you can purchase the phone at 0 down? How does that work? It sounds to good to be true. I can't imagine it would be 0 down.

Added (1). It says $0 up front in big bold letters

lol dont be stupid