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?
And are you saying that peroxide in a developing lotion really has no ability to lighten hair?
If so, why is it sold in different strengths depending upon the result you're attempting to achieve?
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?
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?

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?

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.
In fact, I'm more confused than I was before!
I'm befuddled by your 'power'!