They work opposite to each other. The slave will produce an invisible train when activated by atls trigger. This means if attached to a default lowered lift bridge the bridge will open when activated. The reverse slave has the invisible train active, this means if attached to a default open bridge it will keep the bridge lowered and when activated by the atls trigger the train will vanish and the bridge will open. The reverse slave is also useful to make scripted signals work properly on level crossings.
Slave = no invisible train in default
reverse slave = invisible train in default
