In a video, I saw this exercise in the image. You should do for 15 minutes daily to get a straight posture. Is this a legit exercise? I couldn't find anything about it on the internet and I fear it maybe won't help or cause more damage to my back.
It depends on what the problem is. Usually, I believe that there are 2 things you need to address for any postural issues/imbalances. Stretching the shortened muscle and strengthening the antagonist.
In this case, this tool can allow you to stretch your chest (internal rotator) and to a certain extend, allow you to do isometric holds for your rhomboids (external rotator). This will help the common issue of “forward head posture”.
But then again, sometimes there are other muscles that are in play such as your lats (internal rotator). I would recommend that if you’re in pain, seek professional opinion first.