What is this key?

[Verafin]

I have been using BS.Player for a long time now, and I have always experienced rendering issues.
Today, I pressed some keys on the player and noticed that the rendering issues had stopped. I examined carefully what I was doing while doing it, and I found a key on the player's VIDEO sidebar that I pressed, and it did not display a name when I was hovering over it with my mouse.
That key is located next to the word "VIDEO" when opening the VIDEO sidebar with the player's default skin.

Can someone tell me what this key is? I looked online and couldn't find any information about it.

I am attaching a picture of the aforementioned key to this post: