First, I don't want this to turn into a "why" argument. This question is simply about "how".

I'm plumbing my line-lock to lock OUT the backs. I've seen here from a few people that to do this, you need to plumb the solonoid backwards (in is now out, out is now in). My question is: WHY would you have to do this? Does't the solonoid simply interupt the line? or is there something else going on? is it maybe essentially a one-way valve? I just want to really understand what I'm doing here. Thanks guys.