The idea here is that what's happening at the end of the roast is generally more important than what's happening at the start. It's conceptually similar to the profile translation analysis that I've advocated for over the past couple decades, but doesn't require picking out significant temperature ranges in advance (plus there's no reason you can't do both if you want to go for the deeper analysis).
The way you'd use this is scan over until you see significant divergence. If that's happening at a low enough temperature closer to the right side of the graph, you're probably fine, if it's happening at higher temperatures closer to the left side you'll probably taste the difference.