they usually either plug into the airflow meter and give a false signal to the ecu causing unmetered amounts of fuel to be added or plug into the fuel pressure switch and again lie to the ecu causing increased pressure, so either way you will possibly get a slight performance increase but you will probably have unsafe fuelling
you are far better with a proper remap
and also to add my mate had a similar thing on his merc vito and it constantley caused his van to cut out and would only restart after disconnecting the tuning box