Disulfide bonds are strong covalent bonds that help make up the hair protein/structure. Relaxers soften these bonds at a carbon and sulfur link to allow for a permanent straightening of the curl pattern. Permanent wave solutions soften these bonds between two sulfur links to allow for a permanent curl reformation around a perm rod. These two chemicals break the disulfide bonds at two different places within the same bond so they will cause breakage if overlapped. They are not compatible. So relaxers reduce natural curl in the hair by causing a permanent chemical change in the hair. Direct heat from hot tools break both hydrogen and salt bonds to allow for temporary straightening or temporary curling of the hair. "Hard to curl" hair may require working spray/styling spray in combination with heat to hold the curl. Typically, curls will hold longer with "direct heat" curling versus curling with a flat iron. The flat iron breaks only the hydrogen bonds; therefore giving a softer hold.
Extremely hot direct heat can soften the disulfide bonds in the hair to become permanently straighened like a relaxer. Hard pressing is an art form that requires expertise as to not burn the hair out.
Heat is heat whether flat ironing (heating inside out) or direct heat (which heats outside in). I am leaning toward direct heat being safer only because most people can not see the damage from the flat ironing until it completely broken.
When looking at new "infrared cookers" that cook foods at high temperatures without losing moisture. It sounds great for food, and I understand the concept for hair too. However, you can hold a curling iron in your hair for extended periods of time to let the "heat" penetrate through the hair, but you should not hold an "infrared" flat iron in one place for any extended period of time. You may be "cooking" your hair without knowing it.
Wet sets, roller sets, and air-drying will always be the healthiest option for growing straight hair out to longer lengths.