The way these work is it replaces an on/off switch with a relay. The buttons and the zwave both have connections to this relay to turn power on and off. Some even have the ability to do dimming, again both physical and over zwave. Hope that helps!
thanks! thats certainly useful. I'm just starting out. I'm sure everyone looks for the same thing (make sure the lights work even when the hub is down) so maybe it was too obvious to include in the description of the product.