Well I'm pretty sure 0 means disabled, So it would do Software based Phsyics from what I can remember coding my custom cfg. So If you wanted Physic on the 4th core, you would have to use sys_physics_cpu 4.
Here is the video I remember seeing explaining the physics command.
Also, On my Q9450 with my Custom CFG, i see crysis using 50-70%(total) of my quad.
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