There hasn’t been much research done on the subject but I hypothesise that hydrogen bonds are the likely candidates.
Although hydrogen bonds are usually only temporarily changed by heat, high heat is usually also accompanied with loss of protein from the hair strand and this loss of protein is what changes the shape of the hair permanently.
This may explain why many natural ladies who have experienced heat damage are able to restore their curls to some extent by doing protein treatments. If very high heat is used the loss of protein from the strand is likely to be extensive beyond the capabilities of any protein treatment and with the hair so weak, it will eventually break.
If it’s in fact the hydrogen bond (weakest bond) that is altered during heat training, It may explain why some ladies are having better length retention with heat trained hair over their previously relaxed hair.
There are also some who have better length retention with heat training over their previously virgin natural hair but this can simply be explained by less mechanical damage during combing as well as less knotting.
I can certainly see why heat training is a more attractive prospect than using a chemical which will alter your second strongest bond however there are also disadvantages.
Heat training is a technique that not many stylists offer so it is more than likely something you will have to attempt yourself and unless you are very well versed with your hair and are able to recognize the subtle changes knowing when to pull back etc, you may very well be left with an uneven texture with some pieces of hair that remain straight even after a wash.
It is also possible that you may take it too far causing major damage that leaves you with hair breaking in a similar fashion to damaged relaxed hair.
Ok, science lesson over but I will leave you with this;
► If your hair is natural and you straighten your hair on occasion, you are probably ‘heat training’ but to a lesser degree than someone who does it purposely.
► Heat styling tools will and do alter the way your hair behaves whether it’s through loss of protein or some other mechanism. Take from that what you will.
► Not everybody will be able to grow their hair long with a chemical and as such YMMV (Your Mileage May Vary) if you decide to heat train instead.
As always, proceed with caution if at all!