Sorry

Right, as standard it just looks at atmosphere, meaning the fuel pressure is always held constant.

Modified, it will see vacuum and boost. so the fuel pressure will no longer be constant going into the metering unit.... instead on boost the fuel pressure will be higher (richer mixture) and off boost the fuel pressure will be lower (leaner mixture)....

This is good for us, because when we run higher boost on MFI we need more fuel, the standard way is just to turn up the fuel pressure and leave it high.. great on boost but really rich off boost..... The above method of connecting a line from the fuel pressure reg to the manifold helps this off boost overfuelling a bit.

Rob,