Its the new streaming multiprocessor engine that powers the GTX 680.

A fancy name  for the way they packed far more CUDA cores in a gpu than before but lowered the clocks by half.  It works but all in all its just a new jargon term. From the white paper "For improved power efficiency, the SMX now runs at graphics clock rather than 2x graphics clock; but with 1536 CUDA cores in GK104, the GeForce GTX 680 SMX provides 2x the performance per watt of Fermi’s SM (GF110)"

haha  Hey, if they start making games that utilise X2 Precision, I'd buy a Quadro.  and key word, "A"  and would be happy with it for a Looong looooong long time.   hey, some people get (4) 580's or 680's.  which costs the same as a quadro all added up.