The iPhone is usually the item you have at hand when you need to make a quick note, and Knot takes advantage of this by making it as easily as possible to leave yourself a reminder -- tying the proverbial piece of string around your finger.
Knot goes a step further from basic reminders by utilizing associative memory to prompt you to actually remember what you have to do. You choose where or when your phone will alert you that you have a task to accomplish, type it in, then hit the home button to close the app.
When you reach the location or time of the task, Knot will remind you that you needed to do something -- it's up to your brain to supply the missing information. If you actually remember your task, acknowledge it in the app. You can cheat, Knot keeps a list of active tasks you can access at any time. But at the bottom of that list is a memory score letting you know how much you actually remember or shaming you into trying to work those memory skills.
It's great for exercising your brain, but Knot is best used when you're busy and need to jot down a reminder for something that will be done in the next couple of hours, not the next few days. Knot isn't good for long-term planning or GTD gurus, but it's great for remembering those little one-off tasks that pop up during the day.