Defines explicitly what we naturally do when planning anything. David Allen recommends we use it consciously to increase personal and team productivity. If more clarity is required on a project, move up the model. If more action is required, move down the model.
The practice of regularly and completely emptying all of your inboxes so that your trusted system is updated with the outcome of processing all of the new stuff. The opposite is trying to use, for example, your email inbox as a management system in itself.