When you enter the Grand Underground in Pokémon Brilliant Diamond and Shining Pearl, you will notice that you have a counter that says 0 / 40 under your mini map. This feature is not really explained all that well in-game, but most people soon realize that you get +1 when you find a Diglett and +3 to the counter when you find a Dugtrio in the underground cave.
Filling it up to 40 can take a little while, so what does it actually do, and is it worth it? Here's what happens when you hit 40 / 40:
When you get the counter in The Grand Underground to hit 40, you get the text saying "Something good will happen!", and the bar begins to glow. During the 4 minutes it takes to drain, you you be able to dig up special green glittering statues, and have a very increased chance of finding shiny Pokémon when battling wild Pokémon.
Keep in mind that the effect only last a total of 4 minutes, so you have to be efficient when you first get the buff. Hurry up and do some fossil digging or fight wild Pokémon to have a higher chance of getting a shiny one.
The statues you can find while having the Diglett / Dugtrio bonus can be very good, and have effects such as increasing Shiny chance in The Grand Underground permanently, or having increased chance to find Legendary Pokémon.