A Mass Air Flow Sensor (MAFS) measures the amount of air that has passed through the intake. This information is used by the ECU to determine how much fuel should be mixed with the air.
The limit of a stock 240SX MAFS can be mixed with enough fuel to generate about 250hp. Air flow beyond this amount is not properly metered, meaning not enough fuel will mix with the air. This can lead to engine detonation (air/fuel mixture exploding spontaneously in the combustion chamber before the spark plug fires), and can destroy an engine.
One method for modification to a MAFS is to "trick" the sensor, in conjunction with larger flow injectors. For instance, the popular 370cc injectors swapped the SR20 engine will flow approximately 30% more fuel than stock injectors. By increasing the cross-sectional area of the intake piping by 30%, the MAFS unwittingly sends the correct signal to an standard ECU to mix a good A/F ratio. This technique should be used with caution because if you are using a turbo to increase air flow and a modified intake pipe, the ECU will adjust the air/fuel ratio but not the ignition timing and this can damage the engine. Some options to counter this problem are to back off the base timing, or use a product such as MSD's Boost Timing Master to retard the timing under boost.
The safer, and more popular modifications for a MAFS are to swap with other units that can meter more air. This is typically done in combination with an upgraded ECU and injectors. The Z32 MAFS is probably the most popular. The Q45 MAFS is another option, and some have reported success with a Ford Mustang MAFS. Your ECU must be aware of this change. Jim Wolf Tech offers reprogrammed ECUs for Z32 and Mustang MAFS.