It's an interesting fact that the heart rate increases during inspiration, and decreases during expiration, which is why you can see the red dots following the blue curve on the graph

So if you breathe in a more regular way, it's also normal that your heart rate will vary a little less, since it follows your breathing cycle.

There's definitely also an effect where slow, regular respiration can help with anxiety. But I'm not sure how much of what the tool is measuring is really tied to stress response, versus just demonstrating this direct inspiration/heart rate correlation