The Smooth feature of the Muscle deformer lets you apply a smoothing operation on your skin mesh. The effect of this is that points move together and cause your mesh to smooth out and lose detail.

Smooth deformation can be used as a stand-alone feature. For example, even if you are not skinning or using muscles or bones with the deformer, you can apply the deformer to any object, turn on Smooth, and paint smooth weights. This causes your mesh to still use Smooth features.

Smooth can be used to help pull areas that are sliding or to smooth areas in your mesh where unwanted wrinkling or pinching is occurring.

In many cases, Smooth can give results similar to Relax and is usually faster. In other cases, you can use Relax for things like wrinkles, then use Smooth in addition. It is recommended that you use either Relax, Smooth, or both when dealing with areas that slide.