The input is attached to the 'Common' (Line/Load designation on an HS210) and the Outputs are Traveler 1 and Traveler 2. So unlike a typical Single Switch, there is no 'Off' per se - the 3-way Switch redirects the Line Voltage to one or other terminal. Here's the simple part - an HS210 in regards to common switching, is no different from a dumb ... More @Wikipedia
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