Without pairing with the Smart Switch part, the Smart Relay performs as an independent device that adds wireless(HomeKit Wi-Fi) connectivity to your lights and turns regular light bulbs into smart controlled lights. And the Smart Switch is an optional add-on that connects the Smart Relay and your wall switches for expanded functionality:
No matter the physical wall switch is on or off, the power is constantly on at the homekit light switch for it to remain always connected and online, thus you can always access and control your connected lights via apps or voice commands wirelessly.