By doing what the Church does, they don’t mean that the Church is the one actually doing the Consecration, but rather that the Priest is trying to do whatever it is that the Church intends that its priests do. Take, for example, baptizing a person. A heretic can baptize. If he uses the proper form, and intends to do what the Church does, even if the heretic views it as nothing, the baptism is valid. What the Church does is provide the Sacraments, and the Grace, as all Grace flows into the world through it. Here, the Church provides the Mass, even though the actual ceremony is celebrated by the priest, acting in persona Christi. The priest doesn’t have to understand what the Church does, believe it, or anything of that nature. It only has to, overtly, be his intent to provide that thing to the faithful.
The Church teaches that as long as the priest puts no overt obstacle in the path of "doing what the Church does," the Sacrament is to be considered valid. Something overt would be, for example, a priest who is teaching a group of seminarians how to celebrate the Mass would overtly indicate that the external motions, though looking like a Mass, are not a true Mass, but merely for teaching purposes.
patent +AMDG