This is a similar idea to how I understand the Elephant Carpaccio exercise by Henrik Kniberg & Alistair Cockburn (2013), from what I've been able to Google. The key idea is that work should be broken down into "vertical slices" where vertical means that the entire user story is captured, or as it's described at https://uploads-ssl.webflow.com/5e3bed81529ab12a517031ab/5ec..., "very thin slices, each one still elephant-shaped." The first vertical slice might be a mockup or very-low-fidelity prototype of the complete project and subsequent slices are enhancements following user stories. Horizontal slices might be, say, system components or other subtasks that leave you without something prototype-looking until all of the slices are complete. At least, this is how I've interpreted what I've read about it.