Found this on NP. It's taken from another source. The author is responding to a question about damage:
However, as you witnessed, this process is not without risks, especially on fine, fragile hair like yours. Pressing combs typically have a temperature range of 350F to 500F, which is intense heat when you consider that water boils at 212F! The more resistant your hair is to being straightened (as is true for most African-American hair), the more heat (and tension) is needed to achieve results. What can happen when the pressing comb comes in contact with the hair, especially if you leave it in one place for too long, is that the water inside the hair shaft literally boils, which causes the hair to break or rupture in that area, resulting in patches of frayed, broken strands. It also burns away the hair shaft's protective cuticle layer, exposing the fragile cortex (the inner portion of the hair strand) and making it more vulnerable to damage. And this type of repeated damage to hair adds up quickly.
