When the straightening product says you can distribute the product in sections as you straighten wat does...?

like i just bought the herbal essences dangerously straight flat out finishing spray. and it says i can use it with the flat iron in small sections at a time. does that mean i spray it on. before i straightenthe section. [but wont that hurt your hair more? cuz the water sizzles]. or like right after i straighten it, i spray it on the section