I - Initiative; one of those "mysterious" concepts, but usually you recognize it when you see it.
A simple system for evaluating each element of KIMPLODES
KIMPLODES offers a structured way to think about your game.
Basically, I look at each element of KIMPLODES and use my own variants of standard chess notation (reminder below) to make a quick assessment. Then you can subjectively "add them up" (I don't recommend that), or you can determine what you want to do based on which elements you determined are the most important in a given position.
+- White has a significant, perhaps winning edge; +/- White has an edge; +/= White has a slight edge; = neither side has an apparent edge; or "U" for unclear (when I just don't know)
-+ Black has a significant, perhaps winning edge; -/+ Black has an edge; =/+ Black has a slight edge; = neither side has an apparent edge; or "U" for unclear (when I just don't know)
You can make up your own notation, I just recommend keeping it simple. It works during games or analysis (at least, it works for myself and some others I've introduced to the idea).
Initiative: basic building blocks.
Why are we looking at a board before the first move? Technically speaking, the first move grants White a theoretical sliver of an edge because he/she is the first to enter the battle field by moving a pawn or a Knight, to which Black must respond. At it's most oversimplified, the initiative simply means that one side is reacting to the other side's moves. So before anything occurs on the board the I-factor is +/=, and every other factor, from King safety to Squares and Space is =.
The point is that even in this beginning position White can decide to either focus efforts on retaining and building on the sliver of an edge with moves such as 1 e4, 1 d4, 1 f4, 1 c4 (moves that also clearly seek to alter the S-factor, Space/Squares), or seek to start creating other imbalances by playing moves such as 1 g3 or b3. And on every subsequent move, both players are making tradeoffs between all the factors that comprise KIMPLODES, determining whether the M-factor (Material) outweighs the K-factor (King safety) or the I-factor (Initiative)...and which factor(s) the player wants to focus on, as well as determining what their opponent seems to be focusing on.
Initiative in the opening
MCO-15 points out, correctly (obviously), that White is granted a sliver of an edge by Black's choice of opening, but that sliver will not in and of itself translate into a decisive edge for White unless Black plays poorly from this point forward. At the top levels this opening can disrupt an opponent who has not prepared for this opening, and sometimes that is all the edge the Black player wants - avoiding prepared lines in more extensively played and studied theoretical duels.
Here is a case where White sacrifices Material (factor is -+ because Black is 2 pawns up) for a lead in Development (factor is +/- because White has both Bishops developed) in order to gain the Initiative (factor is +/-, but White must exert great energy or risks losing due to the Material deficit). One might even suggest that the King factor is +/-, since White is only 2 moves from being able to castle, and has both BIshops pointing in the direction of Black's King. Generally Black returns some material to blunt White's Initiative (showing how important this factor is), but this is a good opening to look at for understanding how various factors in KIMPLODES relate to and interact with each other. Much of the art of chess involves making tradeoffs between the factors discussed in KIMPLODES.
Initiative in the middlegame
First, let me offer as a given that the I-factor can be of either a positional or tactical nature. I will not be providing examples to distinguish between the two, as that would require multiple blogs or one overly long blog.
Second, I acknowledge that a game where neither side has the initiative in the middle game is quite possible, and in fact very frequent, but I find those fall into two catgories. First, we have IM and GM draws. There isn't much going on, for whatever reason, and neither side can create an imbalance that doesn't unduly jeopardize their chances. Second, we have all the rest of us non-titled players, who just muddle around until one side blunders (and then usually the other, with multiple errors passed back and forth), until finally the carnage accumulates and one side has such a significant material advantage that discussing initiative is irrelevant to the fact the game is over.
Third, the initiative at the titled level can be difficult to follow, one might even call it capricious. So I will present one of my own games where following the initiative is quite straightforward and provide my assessment of the I-factor throughout the course of the game.
In this game the I-factor is the most important criteria against which to judge the game. White first relinquishes the advantage of the first move by his 4th move, b2-b3, so that the I-factor is =. This is NOT an error, just a decision, but it can indicate a non-aggressive mindset, and in this case it was an issue I had detected in prior rated games against Alex, and therefore a key point of our discussions during this training game. By move 9 the Initiative has slipped into Black's hands, and shortly thereafter White is losing a pawn (Material factor -/+), but by sacrificing that pawn in the correct way, resumes the initiative for the next 8 or 9 moves (+/-) and generally makes good use of his time, while Black seems to drift a bit from moves 14-18, simply reacting to various White threats. Finally, Black starts fighting back with 21 ... f5, and within two moves White starts to lose his grip on the I-factor...the I-factor could be described as either = or uncertain after 23 f2-f3?. After move 26 ... Ne5 the game has reached the tipping point, and with a nice sequence of moves Black is able to eliminate all of White's major threats (particularly with the trade of Queens), and a few moves later White resigns as the I-factor has slipped into Black's hands (-/+) and the two pawn deficit grants Black an overwhelming Material factor (M-factor) of -+. Finally, the K-factor is swinging against White (-/+), as now his King is threatened by the Black Bishop on b7, Black Knight on e2, and a potential doubling of Black Rooks on the half-open h-file.
Initiative in the endgame
Inititaive in the endgame is also very important, but there are multiple treatise's on the topic. Below I recommend a specific set of WIM energia blogs on chess.com as one good source on this topic.
The best reason to study the I-factor in endgames is because the reduced forces on the board simplify everything and it is often readily apparent how other KIMPLODES-factors change over time simply due to which side has the initiative, and what the consequences are if the I-factor stays firmly in the grip of one player, or if their opponent is able to grab the initiative for themselves.
WIM energia, Initiative in the Endgame blogs on chess.com
IM Bryan Smith, Attack and Defense blogs on chess.com
IM John Watson, Modern Chess Strategy
GM Mihai Suba, Dynamic Chess Strategy (advanced material)