The Relax feature of the Muscle deformer lets you apply Relax and Wrinkle effects to the skin mesh. The effect of this is that as points stretch or get farther apart, Relax tries to keep them back at their original relative distance. As they get closer, Relax attempts to cause wrinkling to make them stay the right distance apart.

In addition, Muscle offers separate Wrinkle and Relax weights, so that as areas compress, you can get predefined wrinkling to occur.

Relax, like Jiggle, 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 Relax, and paint Relax or Wrinkle weights. This causes your mesh to still use Relax features.

Typically, you want to use Relax around any area that slides, so that the slide effect pulls up the skin around it as well as where it has slid the mesh.

When you paint Relax weights you are setting what points on the mesh might be adjusted after the basic skinning and sliding has occurred. These points are either pulled along or pushed out in a wrinkling fashion. Any point weighted to 0 is unmoved by the Relax/Wrinkle, although points around it may be affected.

By default, the Relax weights default to 1.0 for the entire mesh, so you can simply enable it in the deformer settings to turn it on. Wrinkle weights default to 0, so that you do not get any wrinkle effects unless painted.