There are two basic types of hybrid designs. Older hybrid systems use a multiplexor (mux) to switch between GPU's. Newer systems (those with PowerXpress >= 4.0) are muxless. As far as I can tell, PowerXpress 4.0 started with RadeonHD 6000-series GPU's, and systems with older ATI GPU's have a mux, but don't quote that.
=== ATI/ ATI Hybrids === As of Catalyst 11- 8, switching between two ATI cards (and maybe Intel/ATI muxless too ?) is supposed to be doable, though I don't know if that applies to all ATI/ATI hybrids or only the muxless ones. One would use amdconfig's PowerXpress options to switch back and forth between the integrated and discrete cards, like so: amdconfig --pxl # List current activated GPU sudo amdconfig --px-dgpu # Activate discrete GPU (High-Performance mode), must re-start X to take effect sudo amdconfig --px-igpu # Activate integrated GPU ( Power-Saving mode) , must re-start X to take effect After switching, one would log out and back in to restart X.
=== Intel/ATI Hybrids ===