The spine’s value is the phrase "because of that", repeated. It forces causality: each event has to be caused by the previous one rather than merely following it. Most weak stories, and nearly all weak corporate ones, are chronologies where "and then" would work as well as "because of that", and the spine exposes that in about two minutes.
It is a diagnostic instrument rather than a template, and it is the fastest one there is. Eight sentences, filled in honestly, will tell you whether a story exists before anybody has written a treatment or booked anything.
Where it breaks down is on structures that do not run on causality — kishōtenketsu in particular, whose turn is deliberately not caused by what precedes it. That is a limit of the tool, not a fault in the story.
What is the story spine?
A sequence of sentence stems — once upon a time, every day, until one day, because of that, because of that, until finally, and ever since then — that forces each event to be caused by the previous one.
How do you know if your story is just a chronology?
Try replacing every "because of that" with "and then". If nothing is lost, there is no causal chain and you have a sequence of events rather than a story.