It would probably be a switch that heats the water to above 55°C.

* 70 to 80 °C (158 to 176 °F): Disinfection range
* At 66 °C (151 °F): Legionellae die within 2 minutes
* At 60 °C (140 °F): Legionellae die within 32 minutes
* At 55 °C (131 °F): Legionellae die within 5 to 6 hours
* Above 50 °C (122 °F): They can survive but do not multiply
* 35 to 46 °C (95 to 115 °F): Ideal growth range
* 20 to 50 °C (68 to 122 °F): Legionellae growth range
* Below 20 °C (68 °F): Legionellae can survive but are dormant

(Wiki EN)

http://www.freepatentsonline.com/EP1094279.html
Management of heated water in a storage boiler with anti-Legionella provisions

http://www.sht.at/index.php?id=76
Anti-Legionella function



Or maybe a clever water-shifting device…

Anti-Legionnaire's disease ON/OFF control