It just reads maps from the daughterboard instead of the maps on the stock ECU.
Rev limit, speed cut etc can all be removed/changed.
I believe this is what Abo Bob has got.
Silvia boys have been doing this for ages.
Only drawback is you need an eprom emulator to tune real time and use map tracing and also a writer.
I think the both will set you back around $500?
There's plenty of free software out there written for daughterboards.
Here is one
http://www.925style.com/editor_direct.html
Taken from
http://www.sr20tuning.com/theoryandpartsrequired.html