If you're like most productive people, you're probably getting through several things on your to-do list, but not making progress on that one thing that you know is most important (and usually hardest).
Somehow that thing always gets left until later.
So it always seems like you're being "productive", but if you're distracted by a long list of priorities, are you being "effective"?
With "One & Done", the distraction of a long to-do list is gone. There's only one thing, and that's the thing you know you've got to get done. Your top priority.