Sitepath diagramming - or "eyeball diagramming" as it's also known, and for very obvious reasons as you'll realise when you see one of the diagrams - is a quick and dirty, customer-centric, qualitative take on process modelling.
This technique is less analytic than process modelling, and more synthetic in its attempt to visualise coherence amongst the solution space of an unmapped proposition. In my experience, its a good flip-chart technique, though it's likely to generate a lot of different "eyeballs". So there is a synthesis job to do after you've generated the diagrams themselves.
I've used sitepath diagramming to be quickly and easily visualise user paths and journeys through a proposed site or service in the early stages of a project. In this sense, it might build upon prior work done at the level of the proposition (and the content taxonomy) for example.
Depending upon how the diagrams are physically created, they can also be used to compare different kinds of user and different kinds of task, highlighting commonality and picking out novelty. And if there are personas and use cases available, then the technique becomes even more powerful, because it has segmentation to help narrow down the possible solution space of paths.
Sorry. Did I just get a bit IA-nerdish there?