Tyrek

I'm new to this forum and I'm also new to the auto performance world. I understand how a turbo works and the effects of having a small or big turbo. I also understand twin turbo setups and the reasoning for sequential turbo setups, what I can't understand is how they work. I can't understand the piping systems and what happens to the smaller turbo when the larger one starts running at full capacity. What does it mean when people say that the smaller turbo goes on a vacuum and how do I chose which two turbos to get for a specific engine? Thanks in Advanced...