"Flow is a mental state of operation in which the person is fully immersed in what he or she is doing, characterized by a feeling of energized focus, full involvement, and success in the process of the activity. Proposed by psychologist Mihaly Csikszentmihalyi, the concept has been widely referenced across a variety of fields."
Alister Cockburn's Team Per Task, "It takes about 20 minutes to reach this state of flow, and only a minute to lose it. Our designers found that it took about an hour to get into flow and make progress after having been stopped. If a meeting or other task arrived during this hour, the entire period was essentially lost. As it also took energy to get into the flow, a distraction cost but energy as well as time."
Pairing to to the rescue (again). When a pair gets into a flow it's more resilient to distractions - it seems to allow one person in the pair to get back into it more quickly after being distracted. Maybe it is related to mirror neurons (video).