There are lots of things that engine management systems get put in charge of. There can be so many in fact, that the onboard computer isn't called an ECU (engine control unit) any more.
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The ABS (anti-lock braking system) and the automatic transmission were often brought under the management of the engine's computer, as were AWD settings and later features like stability control, lane-assist and the rest. So, since about the early 2000s we've seen the majority referred to as a PCM (powertrain control module) instead.
It has also been possible to alter the parameters in factory engine management systems for a long time. Some needed an aftermarket (custom-flashed) chip plugged in to take over, while others were reflashable (their memory was re-programmable) even back in the 1980s (for those who had the right hardware and software to do that job).
Some are not able to be reflashed or chipped though, so another option was (and still is) to run a piggy-back system, which either intercepts and modifies the signals that the factory computer is getting from the sensors, or it modifies the signals the computer is sending out (or sometimes a bit of both, depending on the unit or the parameter it wishes to interfere with). This is less ideal and presents various limitations, but still gets the job done in many cases.
Another option, the one that you'll find in many race cars, is a stand-alone aftermarket ECU doing all the work. This is usually the superior option, but often the most expensive too (compared to retaining the engine's original computer for the project and just chipping or reflashing that).
I myself have been more of a budget competitor though, which means that my experience so far has been with the reflash methods (either a plug-in chip with my parameters on it, or just a reflash of the factory PCM).
Either way, remapping is just a question of putting different numbers in the tables, and starting with the factory numbers also makes it much easier for amateurs like me (the manufacturer spent loads of time and money coming up with safe reliable parameters to minimise the emergence of warranty issues).
You may have seen the story I did explaining that boost management isn't just about maximum attack but usable power delivery. That was just changing numbers in tables.
You may have also seen the one where I explained how I converted a couple of cars to biofuel. Increasing the fuel delivery, and the optimisations like adjusting the warm-start temp and the ignition timing, were again just a matter of changing numbers in tables (and a lot of testing before I was happy).
As such, the interfaces I've used have been no harder to operate than a spreadsheet. Plus I take a little old PC laptop with me and the reflash hardware just in case I need to make any final tweaks while we're at the track.
However, you do need to know how an engine works to know what to change, and just as importantly, what to leave alone. Basically, if I don't need to change it, then I don't.
When it comes to engine output, what you're messing with is cylinder pressure (and to some extent, temperature). Whether it's tweaking the air:fuel ratio under certain conditions, altering the ignition timing, changing the cam phase timing or modifying the target boost from the turbo, the resultant cylinder pressure is what gives you engine output, or makes the whole thing self-destruct.
Too much cylinder pressure too early in the combustion cycle (basically, the fuel going bang before the crankshaft is ready for the piston and conrod to come back down in the cylinder) is the shortest path to disaster. Having the combustion occur too late though and you're increasing the temperature (especially in the exhaust) for no benefit.
There are also a lot of tables to look through. The base tables are for mapping out the starting points, and then there are adders and multipliers (although they can be used to decrease the value if you use a negative number). These will make small changes depending on what you're asking the engine to do right now. One example is throttle tip-ins. These are used to temporarily increase fuel delivery or adjust ignition advance (or another parameter like cam phasing) when the throttle is being depressed.