it is a relay on the carnetix unit itself. they call it the ACPI relay because it would be hooked up the ACPI header on a desktop board. on a laptop, the ACPI relay wires would be connected to the laptop's power switch leads.
for turning on the laptop, I suppose in the end it does the same thing as Webmaster's turn on module (a relay shorts the power switch leads for a split second), but the method that triggers the relay is very different. the webmaster module does not turn the laptop off.
the carnetix unit will startup AND properly shutdown the laptop automatically. the SSC is fully programmable and you can set the off delay time, etc.