iMac interface with Embertec Smart Plug

The Victorian Government is distributing Embertec http://www.embertec.com/ Smart Plugs and AV switched that are designed to turn off standby power. For a computer, the Emberplug has two sockets, one is a black "power always on" socket for the computer, and a green section where peripherals are plugged into a power-board which can be switched and monitored. The idea is that when the computer no longer draws current, the devices plugged into the green monitored section also switch off.

The Embertec Smart Plug is also intended to work like this in sleep mode, so when the computer is asleep, powered speakers, USB hubs, printers, external drives atc are also shot down approximately a minute later, but on my 2007 Intel model 24" iMac, a minute after the iMac goes to sleep, the Emberplug switchs off the peripherals, but this act wakes the iMac uo from sleep mode and then everything springs back to life again. Then the iMac goes back to sleep in line with its Power Save settings and the cycle bigins again.

Isuspect that even in sleep mode the iMac is monitoring external influences and when the USB hub switches off, it wakes the mac up to tell it bye bye.

Does anyone have some idea of what's triggering the wake-up call?