The mod comes with 3 funtions:
Xray-> "x" key
Brightness-> "c" key ; Press once to highlight mobs or press twice for everything. 3th time resets to default
Cave finder-> "v" key
You can change the hotkeys in the Controls menu(Yes, the games one).
You can change the blocks you want to show/hide in the menu(ctrl+x).
Red - hidden
Transparent - shown
You change it with ENTER.