Servos apply certain mechanical functions. For instance, in an automatic transmission, if first gear is actuated by the application of a band, the servo is what applies that band. The servo is nothing more than a mechanism (either hydraulically, vacuum, or mechanically operated) that is connected to a mechanical linkage, that when actuated by it's governing force, moves whatever it must to perform a function, when the need for a certain application arises, the servo makes that application.