Flow in software development is delivering a steady flow of valuable software by engaging in all the activities of development simultaneously. The practices of XP are biased towards a continuous flow of activities rather than discrete phases.
Software development has long delivered value in big chunks. "Big Bang" integration reflects this tendency. Many teams make the problem worse by tending to respond to stress by making the chunks of value bigger, from deploying software less frequently to integrating less often. Less feedback makes the problem worse, leading to a tendency for even bigger chunks. The more things are deferred, the larger the chunk, the higher the risk. In contrast, the principle of flow suggests that for improvement, deploy smaller increments of value ever more frequently.
A few trends in software development buck the concept of bigger batches. The daily build, for example, is flow-oriented. However, daily builds are a small step on the road to flow. It is not enough that the software compile and link every day; it should also function correctly every day or, better yet, several times a day.
I visited a team that used to deploy every week. It had more and more problems, until it was taking six days to deploy a week's worth of software. The team chose to deploy every two weeks. This amplified their integration and deployment problems. Any time you move away from flow, resolve to return. Resolve the problems that disrupted your flow and get back to weekly deployment as soon as you can.