it doeesnt actually push the fuel into the rail, since it is connected on teh return line..so is it that it just pressureizes the tank, which the forces more fuel thru? or is there something im missing here

 

It's like a fuel pressure regulator. It restricts fuel in the return line, so it builds up in the feed line. How it does it mechanically though, I have no idea.

 

The less vacuum, the more fuel.

 

Pretty sure it just uses the boost to push on a disc that the fuel passes which gives it less room to flow past, thus creating more pressure.

 

A FMU has a chamber in the bottom of it that allows fuel to flow through it, and there is a metal slug that sits right over the fuel inlet. Then there is another chamber on top of that where there is a plate and a diaphragm that sits on top of that slug. As the pressure builds up from the turbo the air coming in through your vacuum line on the FMU pushes on the diaphragm, putting pressure on the slug, which makes it harder for the fuel to flow through the fuel inlet in the bottom chamber causing a higher fuel pressure between the FMU and your fuel pump, which is where your fuel rail is so it forces the injectors to spray more fuel as more boost pressure is added to the FMU, well at least to the max flow rate of the injectors. Hope this explains everything.