This is a really good question. We use heat to activate the bond building part of the treatment, which allows it to infuse with the sulfate bonds in your hair and strengthening them. To get an even temperature we use flat irons to do this, which results in straight hair on the day. Once you wash it (after 48 hours) your hair will have volume and body, or any other style you want.  If you have curly hair, your curls will be back, just softer and more defined.