Tuning for dummies

[Andrew S]

What can a Chip do that a Flash Tune can't ? Not a trick question, I really don't know.Apologies if not in the right forum but with so many choices.....

[Franco Cozzo]

In the case of diesels, the 'diesel chip', all it does is fool the ECU into thinking rail pressure is lower than it actually is, and the ECU will raise pressure even further.

Personally I wouldn't go down that path, remap vs chip, remap any day as they can change fueling properly.

[Mont5.0]

Not much. Flash tuners are a lot more complex than a chip.

[happy1]

Some of the older petrol models, probably also your AU, couldn't do flash tuning of the original chip. Instead some companies specialized in selling replacement chips that gives more fueling, and different ignition timing etc.
20 years ago companies like Superchips had their chips for many models. These days it is probably just flash tuning of the manufacturers processor instead. The flash tuner loads new instructions into the memory of the ECU / PCM, and when the car is running it uses these altered tables to add more fuel, boost, and change the ignition.
As mentioned above, I never understood why mess up your car DIY at home when there are good dyno tuning centres that gets a much better result. Yes it costs a few hundred more to have it done professionally, but the combustion ratio will be safe, avoiding knocking, under-fueling, damage to your catalyzer etc.
For a while it was popular to have piggy-back computers. These were used on WRX and BA Turbos etc. in the early 2000s. Benefit was to be able to remove them quickly, and also back then the tuners had not yet cracked the access codes / check sums needed to do flash tuning. Disadvantage of the piggy back computers were that they were only overriding the original input to the ECU, instead of writing changes to the program inside the ECU itself.
Cheers