Coolant Tank- not stupid, just ignorant about some things

[RBrummer]

I and my indie mechanic are chasing a small coolant leak under the right rear.

I would like an explanation of the Boxster cooling system that an overeducated farm boy can grasp.

The tank in the trunk is pressurized- pretty obvious

Since it is under pressure, if that tank has a leak, the leak should occur all the time if the engine is warm and you will have a wet trunk, yes?

However, if you have a bad or incorrectly seated cap, it will dump a load of coolant AFTER you shut off the engine (personal experience). what is that all about? Why not as you are driving a warm car? Is that a feature Porsche added so it will only dump fluid after you park it (I am being a little sarcastic here)

There is also a small black object with a relief valve on it. What is the point of that. If open, is it depressurizing the whole system or just the coolant tank (assuming the coolant tank is somehow isolated from the rest of the cooling system while the car is warm and running)?

I have heard on some blogs, that some people keep this valve open for a few days to "burp" their system. If that is also part of the pressurized system, wouldn't it bleed pressure and leak fluid all the time (after the car get warm?

Anyone who can enumerate (an eighth grade word- thanks Mr. Stuckey) and explain the various parts of the cooling system in the trunk and what a failure in each of them looks like would be a hero to many, or at least to me.

Thanks