An FUM goes on the return line back to the tank.  Not the pressure side.  It is basically used to back the fuel back into the cylinders by closing up the return to the tank.  I have used them to make 165cc injectors flow as much as 300cc injectors.  But it does not beat properley sized injectors.  It can also be used to compensate mechanically for an ECU that does not understand boost.  Say adding a turbo to a NA car with an NA ECU.

Jake