How does OS X decide when to switch graphics processors?
On a high level, it's like an automatic transmission - when you press the gas gently, the throttle increases but when you stomp on the accelerator pedal the engine downshifts the transmission to increase the mechanical advantage.

You can influence switching to the powerful GPU by hitting the API with significant work and expect the "transmission" to dispatch that load to the non-integrated GPU.

The documentation on the API are all in Xcode (and online too, but I prefer using search on Xcode to hunt for a document).

I would start with:


xcdoc://?url=developer.apple.com/library/etc/redirect/xcode/mac/1153/technotes/tn2335/_index.html (online web link of the same article)


Those links cover the Mac Pro and the "What you should know when using OpenCL" section contains a helpful note that WWDC 2013 Session 508: Working with OpenCL describes how to switch between two GPUs in a Mac portable using power considerations.