a fifth injector can be independently mapable with a boost pressure switch activating at a certain psi, then connected to a map sensor/crank sensor/tps to allow different volumes of fuel to be injected as the revs go up, this is adjusted through potentiometers assigned to revs at 1000rpm intervals giving an adjustable fuel curve, wouldnt be very accurate on a switch though might need setting up or checking to see if its actually working