The participant chooses any account they want, it could be a random strangers account from the Explore page if needed. They scroll Genuinely thru the posts in any direction, once they are happy, they freely stop anywhere.
You will be able to force them to the exact post or account that you want. You will learn many different techniques which are pretty easy to do anywhere and anytime.
Once you learn the method, you will be able to apply it to other platforms, It’s not only limited to instagram