I know that much.
It was around dressrosa where usopp was developing his that the whole haki system started to lose me.
Before, haki was just the purple-ish black coating, then there were "colors". Luffy knocking people out. People seeing into the future. Sanji has haki but for some reason it's fire. (i know this isn't the exact order of things)
I don't get how you can go from one technique to the other. there seems to be drastic differences between what i assume are the "colors" of haki. It just seems like everytime something weird happens someone says "haki" and just leaves it at that.
If all these different abilities where in separate schools of thought and not just under the Haki banner it would seem less chaotic. It reminds me of Final fantasy XIII and how they kept falling back to these buzz words for everything (ex. fal'cie Cie'th...) I played (platinum two of them) all 3 games and still couldn't tell you the difference between those words with absolute certainty.
Honestly i see haki as a retcon and really try to not put it under a microscope. Also I get that there's foreshadowing to these concepts pretime skip, but i just find the explanations jarring at times.