Just to emphasis what has already been said...
The appliance modules turn off and on. They also are truely OFF and don't allow any power through the switch when turned off.
This isn't true. The appliance modules have a load sensor which sends out a small current. This can be very annoying, and thankfully there is a site which has pictures and details of how to mod this not to do that, and make it truly a
ON/OFF.
Say you turn on your appliance with X10. Then you walk by your appliance, and manually turn it off. Well the sensor current can tell you did that, and it will put the appliance module to OFF, to match the appliance. Problem is, sometimes this sensor causes the module to turn back on by itself, after you just turned it off. real pain. The mod is easy, just cut one wire, now it's ON/OFF and that's it.
http://idobartana.com/hakb/Modifying...nce_module.htm
Scroll down to the "Defeating current sensing"