Activate the 9400 graphics in a unibody macbook pro on bootcamp windows

The Macbook pro unibody has two graphic cores: the 9600 and the 9400. When running OS X you can choose which core you want to use.

How do you disable the 9600 and use the 9400 instead on Windows (bootcamp) ?

TLDR: It's not supported, it's even unwanted and the performance is likely to be poor. If you want to safe battery life, I'm not sure switch GPU's is going to make an enormous difference either. So sorry, but I think it's impossible.

Here's nVidia's take on the problem:

Question

Does the Apple Macbook Pro (Late 2008) support NVIDIA's Hybrid SLI® technology?

Answer

No. NVIDIA Hybrid SLI technology for notebook computers allows a motherboard GPU and a discrete GPU to work together for extreme multi-GPU SLI performance when needed (called GeForce Boost mode), or use just a single GPU for low power consumption and long battery life (called Hybrid Power mode).

Apple's Macbook Pro (Late 2008) does feature both the NVIDIA® GeForce®9400M motherboard GPU for everyday computing and the NVIDIA® GeForce® 9600M GT discrete GPU for high graphics performance. You can switch between the Geforce 9400M motherboard GPU (called "Better Battery Life") and the Geforce 9600M GT discrete GPU (called "Higher Performance"), but you cannot use both GPU's at once in this implementation.

Apple's hybrid graphics technology is supported under the MacOS X operating system version 10.5.5 and higher only. When running Microsoft's Windows XP™ or Microsoft's Windows Vista™ using Apple's Boot Camp, the system locks into higher performance mode which uses the Geforce 9600M GT discrete GPU for all graphics related tasks and can not be changed to use the Geforce 9400M motherboard GPU for battery life mode.

For more information on Hybrid SLI technology, click here: Hybrid SLI

For more information on the Apple MacBook Pro featuring NVIDIA technology, click here: Apple MacBook Pro

Furthermore you have to be using Vista or Windows 7 to even be able to use it:

Q: Which OS supports Hybrid SLI?

A: Hybrid SLI is supported only by Windows Vista. It is currently not supported with any other operating system.

Though Microsoft seems to think Hybrid SLI is a bad thing, because Fudzilla claims that it won't be supported for Windows 7. This makes it even more unlikely it will be easy to find a hack for this...

In Microsoft’s Guidelines for Graphics in Windows 7 document, which was released yesterday, Microsoft outlined its feelings about hybrid graphics, stating that ‘Windows 7 does not offer native support for hybrid graphics systems.’ Not only that, but Microsoft added the reason for the decision saying that hybrid graphics systems ‘can be unstable and provide a poor user experience,’ and that it would ‘strongly discourage system manufacturers from shipping such systems.’ Microsoft also added that ‘such systems require a reboot to switch between GPUs.’

So if Windows doesn't handle the Hybrid SLI very well, then you won't get any performance boost either:

When applied in PC notebook SLI Hybrid technology does not provide the expected performance level so far, as both chipset and GPU spend a lot of time to exchange data and 2D/3D rendering are not really boosted. However, as a recent technology, it will improve over time and its GPGPU power might be important as well.

This isn't possible using Apple-approved means (see Apple's knowledge base article).

You might be able to do some kind of awkward/clever hardware hack to make it work, though for the life of me I can't find anyone else who has written about it being done successfully. Your Google-fu may be better than mine.