I'm helping my son replace a simple three terminal, (two way or "upstairs/downstairs") switch with a motion control occupancy sensor unit. The unit, which takes the place of one of the switches is a PIR 180, (as shown in the diagram at the link I've included below).

The schematic, which illustrates one three terminal witch and a PIR occupancy sensor doesn't seem to make much sense to me. For one thing it has you tying one of the travelers of the remaining three way switch to the common terminal of that same switch. Why not simply replace the switch with a simple SPST, (on/off) switch? And I have another concern as well.

I've looked at this simple circuit until, (I'm embarrassed to say) it's given me a headache, and for the life of me I cannot see how this unit has a return to neutral. We set this up on the bench first, (mainly because I had my doubts as to whether or not it would even work). To my surprise though it did work, and quite well too. The unit loses power however if the load is removed! What???? And although I expected it, disconnecting ground had no effect.

So what's going on here? How is this a complete circuit? Lenny

Link to diagram:

https://www.google.com/search?q=occu...2F%3B532%3B299