The so called Level of Detail BIAS (LOD BIAS) controls at which distance from the viewer the switch to lower resolution mip maps takes place. The standard value of the LOD BIAS is 0.0. If you lower the LOD BIAS below zero, the mip map levels are moved farther away, resulting in seemingly sharper textures. But if the scene is moving, the textures start to shimmer.
Because of this, it's not a good idea to use a lower LOD BIAS to improve the sharpness of the image. It's better to use an Anisotropic Filter instead.
Some games force a negative LOD BIAS nevertheless. The result is heavy texture shimmering. To avoid this, the driver can clamp the LOD BIAS to zero. That means that the LOD BIAS can still be raised above zero, but it cannot set lower than zero.
The default value for the LOD BIAS is 0.0 (zero) and it should be left at its default value in almost all cases.
If you have texture shimmering in the game you can't get rid of with any of the other options, you can increase the LOD BIAS slightly. But note that this will also cause textures to lose details in a shorter distance from the player.
If you want detailed textures at all costs and if you don't mind texture shimmering, you can reduce the LOD BIAS to a negative number.
Direct3d 10 and higher give the application developer complete control over the texture mapping process. If the developer chooses to use mipmaps they can adjust the LOD bias themselves but the driver cannot. This means the LOD bias option in applications like nvidia inspector and nhancer will only work d3d9 and earlier.
This page is taken mostly from: http://www.nhancer.com/?dat=d_optimizations