What the 0 / 40 Diglett and Dugtrio feature actually does (Pokémon Brilliant Diamond and Shining Pearl)
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.