Vertices influenced by a rigid envelope follow the link itself rather than the deformation spline, as deformable vertices do. Low vertex-count models used for games are typically connected to the skeletons with rigid envelopes to simplify export to game engines. In such a case, each vertex can be simply described as assigned to a specific link. It can be located by giving a length along the link, a distance from the center of the link, and an angle around the link. As the link is transformed, all the vertices assigned to the vertex are uniformly transformed. This makes it simple for game engines to move the character's skin.
The vertices move equally with the controlling link, so the skin appears rigid. For instance, in the case of a flexing arm, the skin between the wrist and elbow tightly follows the movement of the forearm, with no change in the spacing between vertices. The skin between the elbow and shoulder tightly follows the upper arm. Vertices around the elbow influenced by both links, smoothly blend to an average position between the two changed links.
Rigid and deformable envelopes can be mixed on a given character. You might, for instance, use deformable envelopes for most of the character, but choose rigid for the head and for a suit of armor around the torso.