Because a Core i7 is more than capable enough of handling PhysX by itself (this is what the tweak does). It allows ALL Cores to be used for PhysX (nVIDIA.. in an attempt to try and claim CPUs can't run PhysX... generally relegate it to a single core). In fact a Core i7 is nearly as powerful (GFLOP wise) in double precision mode as an nVIDIA GT200b Graphics card:
http://www.realworldtech.com/page.cf...WT090909050230. A Nehalen Architecture based CPU is capable of around 102GFLOP in single precision (stock) and 55GFLOP is Double Precision while a GT200b based nVIDIA GPU can handle 622-933GFLOP in single precision and 77GFLOP in Double Precision. My understanding of PhysX is that it uses quite a lot of Double Precision calculations (to calculate physical interactions). So on that front they're quite close in performance. This also explains why enablong CPU PhysX (using the tweak) results in all of the PhysX effect while remaining playable.
I'm playing the game with PhysX turned on (since nvIDIA won't allow me to use the 9800GT anymore for dedicated PhysX) and it's entirely playable (I haven't seen it dip bellow 30FPS with ALL the PhysX candy turned on). To prove the point I will upload an HD video on YouTube... keep in mind I am ALSO recording while handling PhysX and the game.. so the CPU is under a lot of pressure yet does it all effortlessly.
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