I'm afraid I don't know the specefics of it, but recent and upcomming games can run part of the game on one core, while other parts of the game run on other cores.
A 2.0 ghz dual core processor would normally be limited to 2.0 ghz for any one program, since many older programs weren't designed with multiple cores in mind. Now days many programs run with multiple threads, so that they can be split up across different cores.
As an example, let's say we have a complex first person shooter running on a quad core processor.
One core could be processing all the artificial intelligence for your enimes. So they know when and where to move, shoot, etc.
Another core could be processing the environment your in. Trees, water, whatever could be running on this second core.
On the third core everything you do is being processed. Your movements, your shooting, your jumping, etc.
... and the last core, although more of an example of multi-tasking rather than multi-threading, could be running Windows, your anti-virus, services, etc.
Multi-Core CPU's will be the way to go, if they aren't already.