Well, as I understand the reasoning, it's kind of like this.
Imagine you're on a road that forks. Each path leads you into a different cave. According to the signs ahead, one of the paths leads you to an underground oasis of sorts, with plenty of food and water, and you could technically live there the rest of your life. The other path leads to a large magma pit, also according to the signs. Even though you would totally never take the path to the magma pit, you theoretically could. That's what they are calling free choice, or free will.