When herbal essences says that it will straighten your hair but then says for straight hair to make straighter hair what does that mean can you have curly hair making it go straight by using it?

I have never used this product but my friend that I dance with does almost everyday. She puts it in her hair (very curly) as soon as she gets out of the shower (before even putting a towel on it) and then she lets it dry. Once dry she uses a hair iron and irons it straight. Her hair stays PERFECTLY straight all day long. When i just use an iron with any other product my hair frizzes and curls after about an hour. Hope this helps. :)