Mach Speed Technologies TRIO T4300HD - Manual (Page 13)

Only 6 videos are displayed on the screen at once, if you have more loaded on
the player, touch the screen and slide your finger up to scroll the list. To begin
playing a file, simply tap that file.
Button Explanation
Display Mode- This player has a 16:9 screen ratio, and if your
video file is the same ratio, pressing this icon will do nothing. If your
video file is a different ratio, this will alternate between full screen
(video dimensions) and “optimum display” (also by video
dimensions)