The mask modifier is used to hide (or rather delete) verts with a vertex weight of 0. However, this "verts with a vertex weight of 0" is inconvenient when you want to use this for animation. (see https://www.youtube.com/watch?v=fE-uDqBpXxI for an example)

With this patch, you can set the threshold for masking via GUI.
Vertices will disappear if

• the weight is less or equal to the threshold, or
• the vert is not in the vertex group

The following .blend shows how this value can be used with a simple animation.