On the later l series models, it is possible to reprogram the ecu through the obd socket. This process is commonly known as remapping. The basics of the process is that a laptop and programming module is plugged into the obd socket, and the original ecu file can be exported. This file can then be altered, eg, fuelling, timing, boost limits, etc. and then exported back into the car. This process would usually be the last port of call for any car tuning project, as the car is setup to any modifications (hardware wise) that has been changed on the car. The revolution of this reprogramming has taken things forward with power outputs and other features. The most common feature is the mafless map, which came from the problems related with poor performing maf (air flow mass sensor) sensors. Is also gives the opportunity for dual mapping system but at present can only be used in the earlier obd1 cars.
So to sum up, this is the process of reprogramming the ecu for performance and other features.