It's just a capacitor, possibly with a shunt bleeder resistor for safety and possibly with a small series resistor to reduce peak currents and save the switch contacts.
Wired across the lights it will present a low impedance in comparison with the leakage path through the stray capacitance in the wiring and any semiconductor switch (triac). Hence the voltage appearing across the lights when switched off will be too small to lead to any flickering effects or the like.
It's a useful item to know about, with wider application then just the Smiths/Timeguard products. Flickering CFLs, unwanted illumination of neon indicators, etc.
I would have thought that it was just a cap and resistor in series. I must buy one to see what is inside them. I have used similar things in the past on boiler Y plans.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.