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 habit of doing something as soon as it shows up if it will take less than 2 minutes. The idea is that this is roughly the point at which it will cost more to manage the item in your system than to just do it.