Here’s a good explanation from the Roman Ritual
And why are such simple things like the sacramentals so efficacious in the life of grace? Because their efficacy is dependent on the power of the Church’s impetration, and not solely on the devotion of the subject who uses them. We say that the sacraments work “ex opere operato,” that is, in virtue of the outward signs that are posited. On the other hand, we are accustomed to hear that the sacramentals work “ex opere operantis,” which would mean in virtue of the intensity of devotion in those who use them. Yet this is only part of the truth. The thing is cast in an altogether different light when it is stated in full precision, namely, that the sacramentals work “ex opere operantis Ecclesiae,” which means that their efficacy is in first place dependent on the power of the Church’s intercession, and only secondly on the devout dispositions of the subject concerned.