How do external circuits in light switching units (such as Lightwave RF) use the mains wiring for power?

Question

LightwaveRF is a simple switching unit that contains additional circuitry to enable automatic wireless switching. The additional circuit is powered by the mains light switching circuit.

This is my how (I think) a simple single way mains switching circuit works:

So there is a live and neutral present, but for an external circuit to make use of this supply, the mains circuit must be closed (so the bulb must be lit), right? If not, can you explain why? Can you explain how devices like LightwaveRF power their external circuit from an open mains circuit?

I'm sorry if this is a stupid question, I just cannot get my head around how these things work without closing the circuit and turning the lighting on.

Answer 1

A 60W incandescant light bulb requires 500 mA @ 120 V to light it up to full brightness. You can easily pass 10 mA through it — enough to operate a control module like Lightwave — without causing it to light up at all.

This is why such modules are often specified to work only with incandescant (resistive) loads. CFLs and LED bulbs will often glow or flicker on the current that they pass.