Can someone explain to me how a FMU works?

If I understand correctly, it boosts the fuel pressure? So, if that is the case, why does the stock fuel pressure regulator not limit this? Thanks.

The FMU connects to the return line of the fuel system. It restricts the fuel returning to the tank to drive up pressure when it sees boost in the intake system. The FPR regulates pressure entering the fuel rails, so the 2 work together (additive).

What Obe1 says with this addition. The stock fuel pressure regulator is plumbed ahead of the fmu. The return line out of the stock fpr plumbs into the fmu where fuel return is restricted in proportion to boost. FMU's have a diaphragm ratio, such as 6:1. For instance every 1 psi of boost will increase your fuel pressure 6 psi above your fpr setting. so 8 psi of boost would increase your fuel pressure 48 psi. You could conceivably end up with over 90 psi of fuel pressure at max boost.

it basically make a small injector squirt more fuel via pressure. on my promod we run 165# injectors @ 95# pressure instead of 45 since i could not find any larger ones the where reliable.