The flow of battle is something I've been speaking on recently. I feel it deserves an expanded mention, as I believe an appreciation of it is an important skill in Warhammer 40k (...and, frankly, a lot of other things, but 40k has its own understanding).
Defining Terms: What do I mean by 'Flow'?
Each unit has its own rules for movement: Cavalry, Terminators, Rhinos, Land Speeders, and Drop Pods all move differently, but they all still move. They move a set distance a turn [even if that's zero inches for a drop pod on the field], and may do other things like move and shoot.
Each unit, then, behaves in distinct ways. They can do a few actions a turn, but that turns into potentially 7 turns across the game. A unit can start on one end of the board and move all the way across (...in some cases, several times, potentially). Other units start in one spot and stay there all game. Most of the rest fall somewhere in between.
What Does Flow Do For You?
The Flow of Battle speaks to the movement of your units, and your enemy's, across space and time. Basically, an understanding of flow of battle allows you to plan out your next few turns, up to the end of the game. Thus, you can set up a solid battle plan, and can often read your enemy's plan by knowing their units and/or their deployment.
Flow In Armies
Slower armies need more understanding of the flow of battle for deployment. Speed leads to flow; immobility is the utter lack of flow. In other words, the Eldar have a much faster flow of battle because of their grav tanks, while space marines and the Imperial Guard have a slower flow because of their vehicles.
Foot armies are usually slower, unless they are heavy on bikers and/or cavalry.
Note that most armies get (or give up) speed in exchange for other things. I will tell you, though, that it is harder to predict where an Eldar Wave Serpent will be in two turns than it is to figure out where a unit of 5 terminators on foot will be in two turns. Speed gives you options, but does require you to think a little more.
On the other hand, there is nothing quite as fun as maneuvering and denying your opponent his strategy.
More To Come on Flow
I will attempt to break down flow on a more individual basis for various types of units. Frankly, a good, honest look at the flow of battle has to be fairly in-depth, so this series of articles will take time. There will be pictures, and those will speak volumes.
A Final Introductory Note on Flow
I suspect a lot of us out there have at least dabbled in writings on tactics. I have, I'll admit it. One of the things that always kind of miffed me was the occasional line in something like my (probably very abridged, bought on a whim a while ago) copy of Book of the Five Rings. It was something to the effect of "Do this a lot and get good at it."
Man, that line annoyed me. I spent one of my customary christmas/birthday bookstore gift cards on it to LEARN something, not to be told to DO something.
There is a note of truth in that line. Now, I realize it means "learn from experience, boyo." What I write in this and the following articles should be food for thought, but to truly internalize it will take time, effort, and games. You will have to look back over a game, and look ahead in a game to really, truly get it. Once you get it, though, you should be a more effective player.
Personally, I think there's no sin in improving your game.