Originally Posted by Sh0velMan

So because they re-write the core software that runs on the ECU, basically just using the ECU as a computer to run their own operating system on, they can use the various inputs and outputs however they want.

So if they want to use one of the outputs that might have been for something not needed any longer (like some sensor or solenoid that can be eliminated, EVAP or some such, I dunno), to control a boost solenoid, they can.

Oh, and there may actually be inputs and outputs that are available on the ECU, but not wired to anything at all in the stock config, so they can make a change to the harness and use those inputs for other purposes, like boost control.