I don't think you are suppose to comb through because you might make it straighter than you'd like it to be. I only say this is my experience with Just For Me's Texturizer and putting that in my neice's hair. You aren't suppose to stretch or comb through, just apply and let it do it's thing.
I agree with Malice. For texlaxed hair, I've always used lye relaxers & not combed through. One time, I had the stylist smooth with the comb, but I insisted that she not comb through. What I got was smooth hair with lots of body and texture but not stick straight hair.