Can Anybody Explain Me How Compression Works?

Hi guys,

I'm trying to learn Logic Pro in-depth. I've been reading the official Logic Pro Effects manual and, so far, I'm doing well. I understand almost everything; I feel like I am really improving.

But then, compression showed up. I just can't understand how it works. I read the manual, even made a scheme out of it, but I still can't fully understand it. Like, I know that there is the threshold and that when the input goes above it the signal gets reduced, and the ratio controls how much of the signal is reduced. I also understand the Attack and Release parameters, but after that, I'm done.

I was hoping somebody could explain me what the heck is the Knee and what is it used for?

Also, if a Compressor is used to lower volume of a track, why does it boost the volume of every track I compress instead of reducing? I mean, if you set the Gain to -30 dB then of course it will reduce the track volume, but if I apply a normal gain like 5 dB, why does it still boost the track?

I'm sure these sound like silly questions to all of you...

Well, I'd be forever grateful if you could help me with this.

Thanks!