Can you explain Boolean Difference to me like I'm an idiot?

[Daftlander]

I'm trying to use boolean difference to cut shapes out of a mould in Blender, some cuts work flawlessly, but some leave big faces in place while cutting underneath and some make the object that is to be cut entirely disappear. I am losing patience rapidly. I would use the, much better, cut functionality in Fusion 360 but it can't handle changing the complex meshes into bodies. 

 

 

I read help articles, try everything I can think of and this never gives me a consistent result. It's beginning to piss me off.