The youtube player has 3 different bindings for the keyboard arrow buttons. The chosen binding depends on which of the player control elements has focus.
What is focus? The element is called focused if it is activated by the mouse click or via the keyboard when using Tab / Shift+Tab keys. The focused element has privileged keyboard input processing. For example if the play button is focused, then pressing the «space» keyboard button will start the video playing.
In fact all these keyboard bindings bring more confusion than help, as the user usually doesn't know which control element has focus (the indications are very poor) at the moment when he/she/it presses the arrow keyboard buttons.
So these arrow buttons 3 different bindings are:
1. If the sound volume slider has the focus: Left/Down - lowers the sound volume, Right/Up - raises one.
2. If the playback seekbar has the focus: Left/Down - jumps backwards in the video, Right/Up - jumps forwards.
3. If any other control has the focus: Down/Up - changes the sound volume, Left/Right - jumps in the video playback.
What this script does is escape all this confusion: neither the sound volume slider nor the playback seekbar from now on ever gets a focus - therefore there's always the 3rd behavior in action.