I remember reading Lasker's Manual of Chess, especially one page relating about function. That is function of the pieces on the board, their relation to one another for example my rook can take my opponents bishop which however is protected by a knight(function), so taking the bishop right away would be a bad trade for me... not so if I can get rid of the protecting knight first!(by taking it with my other piece) then the function is destroyed and I can take the bishop for free. That is just one example of using the function idea... It was just one page and the biggets Aha from the book for me. Also in-between moves were a step forward in my chess improvement. Wonderful topic.
What were your "Aha!" Moments in your chess development?

After calculating 8 ply deep, and getting zapped on the 9th ply, I switched to universal openings and largely positional chess.
Simple cure, and many less headaches OTB.
Let's you play effectively at higher speeds, too.
abnormally true for you but it is better to have a tactical style at higher speeds

When I started winning tournaments, and when I hit th 1800s in tactics and when people started to call me an inspiring chess player and a good coach. And when I got proud to be SebLeb0210

When I analyzed 4 of my games with clubmates and learnt so much that I won 200 rating points in the coming year.

I have aha moments everyday in the tactics trainer :-D
My biggest positional aha moment was how to fight pesky opponent knights in your camp. Which is not letting them enter in the first place

My biggest aha moment was when I learned to play with myself, chess I mean, not the other thing.
Do you find that you win often?
Seriously, how do you do that?
Seriously, I have 5,371 personalities and quite a few of us like to play chess. So there's a chess tournament going on in my house every night. Join us some time!

Concerning openings, I've found CC helped me just as much as blitz has, only in a different way of expanding my repertoire. On any given CC game, I will devote several hours in opening research alone, and over time, that has allowed me to include openings in my live repertoire. Blitz lets me find weaknesses in my repertoire, as well as getting a feel for typical plans my opponents will likely adopt.
Would you care to spend a bit of time telling me how you do 'several hours of opening research?' What does this mean, and how does one do this?
Since I play CC professionally on ICCF, I end up using large databases on openings. What I typically do is play through dozens of the best quality games I can find in the line I'm about to play. I'll look for what kind of middlegame positions come up and what kind of dynamic play I can hope to reach in those positions. I scrutinize lines that appear to be doing well for one side or another to see if I can determine the cause, as well as any side variants that seem to take the life out of the position. Since I don't want to waste a year-long CC game on a bad opening choice, I try to be as completely researched as possible before starting the game.
One other aspect of my opening research is looking at my opponent's game history. I look for what they generally prefer to play, and then I look to see if there's a juicy variant in their opening that they have not faced yet. It's basically player profiling, though sometimes it can backfire. I remember one opponent I faced recently had ALWAYS played the same variation in the Queen's Gambit, so I spent several hours preparing and researching the best line I could to counter him. We start the game, and for the first time in his career, he played a different safer line! I ended up drawing, but the game was a wash in that I wanted to win.
thank you firebrandx
After calculating 8 ply deep, and getting zapped on the 9th ply, I switched to universal openings and largely positional chess.
Simple cure, and many less headaches OTB.
Let's you play effectively at higher speeds, too.