Yes - it's just a digital spirit level of sorts, that instead of being calibrated to align with the ground is calibrated such that, based on typical angles and dimensions of a human body leads to the wearer holding their arm up in front of them with the device in line with the heart.
Here's what it looks like, the device won't start until you've got the ball to hover in the middle circle, but it has no way of knowing if you're doing it properly or if you're lying down or leaning your body forwards or whatever else would mean that the angles no longer put it level with your heart.
Basically the same logic as, for traditional upper arm cuff devices, giving the instruction to rest your arm on something next to you that allows your forearm to be resting both comfortably and straight, parallel to the floor - which again, doesn't technically mean the middle of your upper arm must be level with your heart, but since nobody would really be comfortable putting their forearm flat on a low down coffee table or a high up standing desk it works as a proxy that's simpler than asking people to think about lining anything up with their internal organs.
Here's what it looks like, the device won't start until you've got the ball to hover in the middle circle, but it has no way of knowing if you're doing it properly or if you're lying down or leaning your body forwards or whatever else would mean that the angles no longer put it level with your heart.
https://imgur.com/a/pV6xcpc
edit to add: so when used correctly it looks roughly like this - https://m.media-amazon.com/images/S/aplus-media/vc/573a171a-...
Basically the same logic as, for traditional upper arm cuff devices, giving the instruction to rest your arm on something next to you that allows your forearm to be resting both comfortably and straight, parallel to the floor - which again, doesn't technically mean the middle of your upper arm must be level with your heart, but since nobody would really be comfortable putting their forearm flat on a low down coffee table or a high up standing desk it works as a proxy that's simpler than asking people to think about lining anything up with their internal organs.