You will want to get the Diglett / Dugtrio bar up to 40 / 40 if you are planning on hunting shiny Pokémon in the Grand Underground area of Pokémon Brilliant Diamond and Shining Pearl games. When you get to 40 / 40, you have approximately 4 minutes where you have a much higher chance of finding shiny Pokémon, as well as a big chance to find statues that permanently increases your shiny finding chances.
The problem with this is that it takes a long time to get 40 / 40, so it might not really be worth it in most cases. However, there are some methods you can use to get to 40 / 40 Diglett / Dugtrio power very fast, and here's how:
To get to 40 / 40 in the Grand Undeground easily, just find a corridor where you can enter a room with Pokémon on each side. The more intersections and bends, the better (Diglett and Dugtrio tend to like these). Now just run back and forth between this path, while zoning into the Pokémon room to reset the Diglett / Dugtrio spawns. This will allow you to get to 40 / 40 in about 5 - 10 minutes, depending on your luck.
When you get the 40 / 40 effect, go ahead and farm wild Pokémon for shiny spawns or try to find shiny statues in the dig sites. Good luck!