The cool part is that when they are really working to get just one thing done, they are completely relaxed. I can watch as someone's attention allows everything else to slip away except the experience they are recreating in their minds. As they do, their facial expression loosens, their muscles unwind, and they have the peaceful look of someone in trance.
They look the way they do when they are fully in the moment, fully attentive to just the one thing they are doing or thinking, to just one intention. It is tremendously powerful to let go of everything except the one thing on which you really want to focus. It is liberating - paradoxically - to let go of everything except the limits of the activity that has the whole of your attention.
Try it. Be mindful of just one thing that is so compelling, so engaging that it occupies all of your attention - maybe for a moment, maybe for several moments. Notice as you step into the experience that you are also relaxing into the experience.
Enjoy the moment.