The term "hardware" refers to the electronic circuitry that the ECU uses to do its job. Some products make it really difficult to figure out what the hardware is doing, like modern phones that are designed to be assembled (but not repaired) by using glue and tamper-proof screws. Other products will take it a step further and do things like cover the electronics with a layer of epoxy making it very difficult to see what is underneath. Fortunately, the Aprilia ECU is kind of old-school and is just a circuit board in a metal box. The circuit board is covered with a layer of conformal coat to keep moisture off the electronic goodies contained within, but the coating is clear and easy to see through or probe with multimeter leads.
Once you can see the circuit board, the task is to figure out what it does. It is not simple, but it can be done.
Some products are released with a complete set of schematics and perhaps even repair manuals. That really helps figure out what is going on. However, doing that costs the manufacturer time and money, and perhaps gives their competitors ideas about how to copy some interesting circuitry that the product might contain. As a result, most products don't ever explain how they work. So if you really want to know, what are you to do? The process of figuring out how something works by taking it apart and looking at the pieces is known as "reverse engineering". To reverse engineer the Aprilia/Rotax ECU (or any other embedded computer system for that matter), you perform the following set of steps:
Identify all the major components on the circuit board
Work out some basic circuit diagrams
Identify the ECU inputs
Identify the ECU outputs
Take a step back and make some general design observations. This may help as you move past the hardware into the software world.
Once the ECU hardware is understood, you can finally move into the ECU software world:
Disassemble the ECU software contained in the EPROM
Annotate the disassembled code to work out the major bits of ECU software
If you get that far, then you can really start messing with things: see the UltraMod project for more details.