This was the first milestone-trick I learned during my yoyoing life. I literally couldn't believe I could do it. Endless hours of struggling to hit the string, and then gradually I became able to consistently hit it. This is one trick that is a real confidence builder once you get it down.
Not only that, but it's the foundational trick and concept for nearly all other modern yoyo tricks. It could possibly be considered the most imperative trick to learn.