Is there a simpler way of saying the above because, as a layman, I didn't grasp or understand it.
I've never heard of a demi colour.
On a scale of 'permanency', is it above or below a semi permanent colour, or just a completely different colouring system?
permanent...demi...semi...temporary. Demi is permanent color ina semi permanent base. Almost think of it as a hybrid of the two. Longer lasting like a permanent color but without the ability to lift natural hair color.


And are you saying that peroxide in a developing lotion really has no ability to lighten hair?
On its own, by itself it isn't very effective at all. It is when it is mixed with an alkalinization agent that it releases al of its oxygen and the lightening process can occur.

If so, why is it sold in different strengths depending upon the result you're attempting to achieve?
Different volumes (10, 20, 30 & 40 and higher - 3%, 6%, 9% & 12% and higher) are relative to the amount oxygen present in the peroxide. his increased "fuel" when mixed with an alkalinization agent will allow for more lift.
I assumed the stronger the developer, the more it 'opens' the hair, so it receives more of the colourant and the more you open the hair, the weaker it can become?
Actually peroxide is acidic and acidic products by their very nature lock down the hair instead of opening it. It is the alkaline part of the color (ammonia or a substitute) that will cause the hair to swell and the cuticle to open. In addition it will also activate more of the oxygen in the peroxide.
Is it not right that if you're only darkening your hair one or two shades darker, you don't need such a strong developing lotion like the ones you get in off-the-shelf boxed dyes? When darkening a hair piece there is never a reason to use permanent color. Developers in most colors that are over the counter ar 20 vol or 6%..the exception being higher lifting blondes.. 10 vol gives you 0-1 level of lift, 20vol gives 1-2 levels, 30vol give 2-3 and 40vol give 3+ levels of lift of natural hair color[/b]

My experience of dying my hair and using an unnecessarily strong developer seems (to me) to cause my hair to lighten again faster than if I were to use a milder developing lotion.
Is this belief completely unfounded?
see above
And before you get upset, please understand I'm not questioning whether you're right or wrong.
I'm positive you're right.
I just want to understand what you're actually saying, but in a basic principle kind of way rather than getting confused by chemical compounds and technical vernacular, which I'm sure for you is everyday parlance.

Obviously if you put yourself in the hands of a professional colourist such as yourself, you would hope they would have your depth of knowledge and you'd leave them to do the right thing and you wouldn't really need to know any of the above.
I'm trying to grasp this as a novice and how to apply it from a DIY point of view.

If I was standing in a store looking at the array of products available, I wouldn't know how to apply any of what you've said to help me make the best decision.
Anything that says it washes out in 20 shampoos etc is perfect for a piece. The reason is, the hair piece is already chemically processed and therefore the more temporary colors act more permanent in nature and the more permanent color act more temporary.
In fact, I'm more confused than I was before!
I'm befuddled by your 'power'!