Most writing instructors want papers that flow, most writers want to write with flow, few concepts in writing are harder to define than flow. If pressed, I think many writing instructors might paraphrase Potter Stewart and note that they know it when they see it. However, simply identifying that a draft doesn’t flow does little to help a writer understand why a paper doesn’t flow. That’s a harder task, one that deserves a lengthy explanation, and in the future, I hope to do that. Until then, here’s one quick way to spot places in a paper that don’t flow.
I teach students to look for three words, also, another, and additionally (and any other versions of these “adding” words). These words mark the entrance of new ideas into a draft, and their presence often means that point is simply being appended without telling readers how and why this idea connects to the previous idea.
Once these points of disjuncture are spotted, incorporating them into the flow of the paper is fairly simple. Writers should ask themselves, “Why is this idea necessary?”, “Why this idea goes here?”, and “What does this idea add to the previous idea?” The answers to these questions should present the reader with a more thoroughly articulated claim that builds on the previous idea in the draft or locates a turning point of an argument. Revising the sentence to tell the reader how and why this idea needs to be added to the previous idea presents the reader with the claim the writer is trying to make. Having a clear progression of claims is the first step to mastering flow.
Good writer, bad writer 
The chapter on “Cohesion” in Joseph Williams’ Style: Lessons in Clarity and Grace is a great resource for discussing “flow” (a forbidden word in my 101 class because, as you point out, it’s such a vague term) as is the chapter on transitions in They Say, I Say by Gerald Graff and Cathy Birkenstein. Both books, for that matter, are handy, practical approaches to writing and are required texts for my 101 classes.