A conditioning agent (like BTMS) is a cationic quaternary compound. It's a positively charged compound that adsorbs to the surface of your hair. (Adsorption means the molecules accumulate on the surface of your hair. It's different from absorption in that it doesn't penetrate, it just sits on top of the hair fibre.) This is called substantivity. This is defined as "an adsorption phenomenon by which materials that have opposing charges or like composition are more readily adsorbed onto or attracted to its surface and, once there, resistant to subsequent rinse-off." The cationic quaternary compound is hydrophobic - "scared of water" - so it will resist removal by water alone. (The more hydrophobic the quaternary compound, the less likely it is to be removed by water alone.) So the positively charged cationic quaternary compound is attracted to your negatively charged hair fibre and clings on to the surface.
Being resistant to rinse-off doesn't mean it won't come clean and cause build up; it just means it won't rinse off when you rinse your hair after applying the conditioner. It will rinse off when you wash your hair with shampoo in the future.
Virgin hair has a pH of 3.7, and it is negatively charged. The more damaged or chemically treated, the higher the pH and the higher the negative charge. This is due to an increase in cysteic acid that forms when the disulphide bonds in the hair are broken and not reformed. So the more damaged your hair might be, the more negatively charged it is. The conditioning agent is going to be more attracted to your hair and is going to adsorb more.
So if someone with virgin hair - are there any adult women who can say that? - uses an intense conditioner, she's going to see most of it go down the drain. You've got fewer places for the conditioner to adsorb due to your low negative charge, and it'll just rinse off.
Cationic quaternary compounds increase the lubricity, static control, and combability (is that a word?) of your hair. It's always a good thing to have extra moisturization in your hair, increasing the water content on the hair fibre. By increasing the lubricity, you're reducing the force required to comb your hair, meaning fewer breakages and less static electricity on the surface.
When formulating a conditioner, we want maximum adsorption and maximum substantivity to get the most out of the product. We do this by choosing a cationic quaternary compound that will adsorb to our hair, like Incroquat BTMS or cetrimonium bromide. The cationic quaternary compound is always the basis from which we work when creating a great conditioner.
We can increase our substantivity by adding a fatty alcohol, like cetyl alcohol, to the mix. Fatty alcohols increase the substantivity of the conditioner by adsorbing to the hair fibre as well and encouraging more adsorption by the quaternary compound.
And we add all the other goodies like hydrolyzed proteins, oils, butters, silicones, and so on to increase the substantivity, adsorption, and moisturization of our hair.
So let's start with the most basic ingredient - the cationic quaternary compound. I use BTMS the most, so we'll start there tomorrow.