How does MPLS label stacking work?

I’ve been looking at MPLS and how it works, but there is one thing I can’t figure out… How the label stacking works. I understand on a very broad level that the labels are stacked because of different protocols, but not why, or how. As far as I can tell the top label is popped and a new one is pushed at each hop. Since this is the case, when would the underlying labels be used? I think that within the label header there is a one bit field for the stack. A value of 1 means the top of the stack, and all other values are 0? I’m very keen to know the nuts and bolts of why and how this works 🙂 please help!