That assumption is wrong. This selector doesn’t tell the server to change the bandwidth of the video.
(the setting for the bandwidth is to be found in Settings - Plex for Mac - Quality)
Instead it only reduces the amount of proccessing and filtering happening in the player.
Some of these filters are described here: HTPC Tips and Tricks
Do also experiment with the “Use Hardware Decoding” checkbox.
I can only assume that this is controlled by MacOS, not by Plex Player.
