Time-boxing is a technique that helps us direct energy on our priorities. When working on a tight schedule towards a goal, we often face tough choices about how to allocate our time. With time-boxing, we set time limits for performing tasks and stick to those limits by eliminating "fluff". Eliminating "fluff" requires constantly assessing whether each step we are taking to complete our tasks is absolutely necessary.
Let's say you're cleaning the floors of your home before guests come over. You want to mop the hardwood floors and then apply a no-odor finish. Your guests will arrive in a half hour. You decide to time-box the cleaning of your floors to 20 minutes, because you still have some other preparations you want to complete before the guests arrive. You then allocate 10 minutes to mopping and 10 minutes to applying finish.
As you start mopping, you realize you won't be able to mop thoroughly within the 10 minute time-box. What do you do?
By time-boxing your tasks, you have forced yourself to face a tough decision. You have to decide whether to do a less-than-thorough job mopping and applying finish, eliminate from your plan the applying of finish, or re-examine your assumptions even more radically. In the end, you might decide to do a thorough mopping of the most visible areas of the floor, followed by applying finish to that same area. If you're able to complete these tasks sooner than expected, you might consider cleaning and finishing the remainder of the floor. No matter what you decide, time-boxing has enabled you to recognize, and adjust for, your scheduling mistakes before the possibility of achieving your goal has evaporated.
Time-boxing works particularly well in conjunction with iterative processes.