Physx runs off your CPU when an Nvidia card is not detected.
If you were to enable the high end physx that the Nvidia cards do, it would crush your CPU to the point that the game would not be playable.
If you have Borderlands 2, you can turn all the physx effects on, if you don't have an Nvidia card, once things get going with the physx, your system will slow to a crawl.