A recent spike in posts about this system led to some PMs and emails from guys working to better understand what is going on inside it. (Unlike a relief valve, where it is clear how pressure is limited, this is not so straight-forward.)

My first idea was to try to get at this through a thread here, but there were some problems with doing that. So I've tried to present the best story I can on this web page instead.

If anyone has a comment about it, I'm hoping there won't be an objection to having a discussion about it here. Fire away with any comments, criticisms or challenges please.