Why the shared screen of the admin will "light dim" after pressing something on the annotation?

[raycheer]

I just finished attended a Zoom meeting for an online lecture from my laptop. I notice when the admin (teacher) pressed the "arrow" of the annotation to start explaining something, the shared screen will be dim automatically itself. I saw something "light dim" appeared, but sadly it does not show up on my recorded screen by Xbox as I checked back. My white chatbox at the side was not dim. My laptop screen was also not dim. However, I was told that it was my laptop's problem that it was low in battery. In fact, my laptop was charging. I didn't unplug the socket. Surprisingly, another viewer said could see the shared screen from the admin. This has happened twice, but I didn't manage to pay attention to it previously for the first one, because it was dim shortly, and then brightened quickly. May I know how to resolve the problem? Me as viewer can use chatbox to communicate with the admin only. Thank you.

[raycheer]

I just tried to go through all the settings, and I found this  "dim screen share video" at accessibility was ticked. This time, I have unticked the box. I hope this will not happen in the future. I am quite disappointed to just discover this. None of the videos provided on the Zoom's  social media has mentioned about this feature. I wonder since when this feature is available?