Sure, since you were mostly polite about it.
Coaches do not affect performance AT ALL. Throw that out.
Coaches do affect the development of learned skills. Contact, splits, throwing accuracy, etc, etc.
Much like cshewood, I think the difference between a 75 coach and an 83 coach is neglible.
However, if you hire a 43 when the average is 78, you will notice a lack of development and/or regression.
Because of this, I believe, like tec, that coaches slow the natural regression of learned skills.
Hope this helps.
Now, had you asked this before suggesting that WifS explain exactly how coaching works, I'd have been happy to answer. Your initial post seemed a bit whiney and demanding. You appeared to be whining because you didn't know and demanding that WifS tell you. I believe you have to learn some things by playing the game. All that said, my opinion of the coach engine comes from 60+ season of playing and observing. Not only my team but other teams in the same world. Am I 100% certain that I am 100% correct? No. But I think I do a very good job of getting drafted players to the bigs.