From LightWiki

Alongside a lot of the values of Dynamics controls is an FX button. This allows you to use a Vertex Map or Surface or Part to control how strong that control is. At the bottom of the map list, there is a Base setting for determining what the 0 value should result in. With a Weight map for example, the normal value (that you set in the control) is used where the weight map is 100%, and the Base value is used where the Weight map is 0%.