Here's an explanation that I copied from another thread. Also check out LadyEsquire's Self Relax Support Thread.
Explanation by Navsgeda
I'm going to give a quick summary. Basically, silk can form a protective covering over the hair to prevent it from being damaged from alkaline materials, such as the chemicals in relaxers. So the next time you relax, try to use something with hydrolyzed silk protein in it (such as CHI Silk Infusion) before putting the chemicals on your hair.
The short and sweet version:
http://www.jajagroup.com/HBADivision/silk_protein.shtml
"After silk protein on the hair dries, it becomes a transparent crystalline protective film. This film can directly prevent the hair being damaged by alkaline materials in hair products. This protective film can increase the hair elasticity and increase the hair’s natural shine. It has a very special protective function in hair products."
But for those of you who need the hardcore science to believe and don't want to listen to me drone on and on:
http://www.freepatentsonline.com/7060260.html