How do they work?
“Sacramentals obtain favors from God through the prayers of the Church offered for those who make use of them, and through the devotion they inspire.” Sacramentals – Good Shepherd SundayFSSP Catholic Priest
Sacramentals should not be thought of as contracts, investments, or good luck charms. To wear the scapular does not give us free reign to commit mortal sin and still be assured of heaven. The scapular is a symbol of Marian devotion and a silent prayer to Our Blessed Mother in heaven for salvation that she most certainly will not ignore. Using holy water is not an infallible wiping away of our venial sins unless we have contrition for our sins when we use it. The power of sacramentals, then, depends greatly on the devotion of both the priest who gives the blessing and the person who is receiving the sacramental. They depend on the prayers of the Church, the prayers of the blessings that are imposed on them, and the merits of Jesus Christ, the Blessed Mother, and the Saints. Of themselves they do not save souls, but they are the means for securing heavenly help for those who use them properly.
Regarding blessed objects of devotion, it is good to remember that it is the blessing the priest gives an object that makes it a sacramental. The blessing gives God ownership over the object and dedicates it to Him, and He then works through it. This is why it is very important to have sacramentals blessed; without the blessing they do not hold any of the graces of benefits promised by the Church. To believe otherwise is to degrade the sacramental to the level of a good luck charm. It is superstition to hold that the grace and spiritual benefit one may receive comes from the sacramental itself; all grace comes from God. A sacramental is merely a channel through which He has chosen to work.