“Flow” is one of those words that communication instructors often use when they give feedback. Its presence makes a reader’s experience easy and pleasant, and its absence causes confusion and frustration. But what is it?
Words and Movement
Think about more familiar instances of flow: traffic, water, ventilation. When we think about these examples of flow, we think about movement; the fewer disruptions to the movement, the better the flow. When we read our eyes and brains “move” across the words, sentences, and paragraphs to absorb the ideas they represent. We want this process to “flow” insofar as new details and ideas follow coherently from the one to the next. Well-designed sentences and paragraphs should do the same for their ideas as a well-designed traffic system does for traffic flow. No one likes being stuck in traffic, right?
Known to New
While we know that a traffic system controls flow with elements like traffic signals and lanes, how does it work in writing? One critical technique, according to Irish and Weiss, is writing from “Known to new.” 
Most documents we write contain both information that is known to the reader and information that is new. Irish and Weiss suggest that to achieve flow writers should aim to assemble information in the order of known to new. Information ordered in this way flows because it is logical and therefore easy to understand. Known information should function as context to help a reader understand the new information they will need to process. However, writers sometimes struggle to order known and new information effectively. It can be tempting to introduce the new information first because it is what both the reader and writer are likely most interested in. But introducing new information without the more familiar context to understand it can cause a reader to become unnecessarily disoriented: the new information will be harder to understand, and the known information will appear in a place where it is not useful.
For more on “known to new” and other principles of flow, have a look at the Irish and Weiss textbook Engineering Communication: From Principles to Practice.
You can find additional tips on using transitional phrases to achieve flow in our online handbook.
 R. Irish and P.E. Weiss. Engineering Communication: From Principles to Practice, 2nd. Ed. Don Mills, Canada: Oxford, 2013.