Turn your iPhone to landscape position ;) (and make sure you are not locking the rotation in control center)

Ah, I see. Very nifty!

We record data from the internal sensors in your iPhone while shooting the video (mainly, the gyroscope), which allows us to estimate how your camera moves when taking the video and compensate for that.

That's a neat way to do it, good stuff!

Blur appears in low light environment, but outside scene look really impressive