What exactly is FullScreen Optimization in Creator's Update?

Question

I was not able to find any documentation or description, official or unofficial, on what the "fullscreen optimization" really does?

It turns "true" fullscreen mode into a hybrid Borderless Windowed mode.

With certain games it is recommended to disable fullscreen optimization. Some reports better FPS, reduced input lag, but the results vary per game, probably per configuration.

Sources:

1. https://windowsreport.com/turn-off-full-screen-optimization-windows-10/
2. https://forums.mydigitallife.net/threads/what-exactly-is-fullscreen-optimization-in-creators-update.74761/
3. https://www.reddit.com/r/pcgaming/comments/bq7kro/windows_10_forces_games_into_a_faux_fullscreen/