A distraction has pulled you away from the breath, and you suddenly realize that you've been day-dreaming. The trick is to pull all the way out of whatever has captured you, to break its hold on you completely so you can go back to the breath with full attention.
You do this by gauging the length of time that you were distracted. This is not a precise calculation; you don't need a precise figure, just a rough estimate. You can figure it in minutes, or by idea significance. Just say to yourself, “Okay, I have been distracted for about two minutes” or “Since the dog started barking” or “Since I started thinking about money.” When you first start practicing this technique, you will do it by talking to yourself inside your head. Once the habit is well established, you can drop that, and the action becomes wordless and very quick.
The whole idea, remember, is to pull out of the distraction and get back to the breath. You pull out of the thought by making it the object of inspection just long enough to glean from it a rough approximation of its duration. The interval itself is not important. Once you are free of the distraction, drop the whole thing and go back to the breath. Do not get hung up in the estimate. 1
Use every distraction as an object of meditation, and they cease to be distractions. 2
Those who regard the mundane as a hindrance to life and practice only understand that, in the mundane, nothing is sacred; what they have not yet understood is that, in sacredness, nothing is mundane. 3