How exactly does Lucidlogix Virtu save power?

[GotNoRice]

when used in "i" mode, does this just keep the discrete GPU(s) idle or does it actually shut them down?

If it just keeps them idle, aren't they basically already going to be idle if you're not in a 3D game? Where is the power savings coming from?

If it actually shuts them down somehow that is pretty impressive but everything I've read so far just mentions that it keeps them idle

I'm considering an upgrade from my current setup to a Z68+Sandy Bridge setup, and my 2x overclocked 4870x2 cards would seem to be a perfect match for this tech.

 

[NaturalViolence]

It actually shuts the discrete gpu off when it doesn't need it. Also the integrated gpu can do some of the lighter tasks like video decoding without using nearly as much power.

 

[Pejaten10]

No, Virtu does not shut down your GPU. Guru 3D has already proved it on their Asus Z68 review. It's just merely 2 W difference.
You can read it more thoroughly here :

http://www.guru3d.com/article/asus-p8z68-v-pro-review/5