The term "passive" refers to a pedal that does not require a battery and is capable of using no power to provide variable resistance. An example of this type is Roland EV5. This is more commonly refered to as an expression pedal or CV pedal. This can only be used for volume if the pedal is designed for volume. Most Volume pedals require the use of an internal battery, are not passive and cannot be used as an expression pedal. ALL expression pedals that are "passive" and are CV type will provide the proper function.