"In reality, fighting is mostly about jockeying for position and trying to create an opening. Most “attacks” aren’t really intended to hit the target - they’re intended to force the opponent to react, get him off balance, and create an opening for you to take advantage of. In many ways, actually hitting the opponent is almost anticlimactic - the real action occurred just before."
I find this to be true in Starcraft 2, as well as Martial Arts & Chess, at which I have much more experience, I will note as more powerful weapons are added to the mix, the more important positioning becomes. Some attacks are so powerful you need to either stop them from launching, like a Nuke, or not be where it lands.
Also as skill increases, more time is spent attempting to gain a position of advantage before you spend resources on an attack. And the less time is spent actually attacking, that is why often high level fights can be very boring to watch for typical audience, they can not even perceive the slight shifting of balance that is going on.
Lower skilled people tend to lack the ability or the patience to wait for, or create, an opportunity to exploit. So they waste resources attacking their enemy's strengths, often in frontal attacks, instead of seeking a position of advantage.
In chess or martial arts when they talk of imbalance, they are talking about trade offs, or opportunity costs, in Chess for example you might open with a gambit. Your sacrificing material (ie you have a smaller army), for a positional (similar to DT/Cloaked Banshee opening) or timing advantage which is similar to a 6 Pool or Proxy opening or a rush in Starcraft 2.
You start out equal, but you spend Material to get something else, so the armies are no longer equal, their army will be bigger, but the other has an advantage that, you hope, will be useful. That is the Imbalance.
You can also spend Position or Time for material, or Position for Time. That creates an Imbalance, just like skipping upgrades will let you produce more units or vice versa.
There is a degree of risk in gambits, but that doesn't mean they are unsound play, for further information look at the Queen's Gambit.
While in Martial Arts, in a general sense, if your attacking your not defending, if you punch with your left arm, it isn't available to defend for a moment of time. Though the reality is more complex than that, since you can defend a punch with another punch.
Similar to a Backstab or Counter Attack in Starcraft 2, your using offensive action as a defense, this is much more efficient than relying on defense. Which is one of the main reasons why skilled players will prefer this technique, another significant reasons is that you can afford to make more of a mistake on the offense than you can the defense.
Since defense is fought near or in your bases, if you mess up you have no time to try another again. But attacks are done near the enemy's base, so even if you lose your whole army, you have at least a little time to prepare a defense.
I will blog more about of Maneuver vs Attrition in future posts, just realize that Attrition is by far the easier to do. Most real wars are decided by Attrition, in part because of the difficulty of Maneuver, but also because the advantages of Maneuver are usually momentary.
Since the advantages of Maneuver usually only last for a brief period of time, you generally use that edge to gain a Material and/or Economic advantage. That is the principle Artosis is talking about when he says "get more ahead".
Creating an Imbalance is something an Enemy can't really prevent you from doing, short of beating you before you create it, so in many ways it is the strongest possible way to play. Since it allows you to direct the flow of the game, though there is nothing that prevents your opponent from "counter attacking" by creating another (additional) imbalance or using the imbalance you created against you.