Using VC++ 6.0, setting this option doesn't seem to do anything significant. Specically, I can still run the app on a 486. I've drilled into assembly and can't find anywhere that Pentium opcodes are being used. My guess would be timing and as I don't have any references to that I can't really be sure.
I would appreciate a short snippet with and without the option being set and a listing with machine, assembly and source.