Unless you just have to have quicksync, don't even bother. It will be of no benefit to you other than making benchmarks appear to run faster (because what it does has a side effect of tricking the frame counter by only partially rendering many frames).
That's the Lucid MVP crap, I don't think OP was asking about that... or were you?
If you meant just regular Z68 Virtu:
In i-mode you connect your monitor to the onboard video output. This will allow you to use the iGPU for anything 2D, along with Quicksync, and your graphics card will sit at idle. It will kick in for 3D. The image is transferred from dGPU to iGPU, and might lose a couple frames per second in the process. Nothing major though. Also, Crossfire/SLI won't work in this mode. It's mainly for the power savings of your card sitting at idle.
In d-mode you connect your monitor to your graphics card. It allows you to still use Quicksync through the iGPU, as well as Crossfire/SLI. There are still power savings, though not as much as i-mode.
From Anandtech review:
To sum up: both modes allow Quick Sync, i-Mode is limited to one dGPU, offers power savings, but at the expense of 3D performance; d-Mode is multi-GPU, no power savings, unless you use Quick Sync over a GPU.
You can check out some testing and results in their review
here.
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