@WouterJ the idea is that when a core member says -1, it means that the PR needs to be changed so that this guy changes his vote, or that the other contributors need to convince him that the current PR is the right implementation.
Keep in mind that a -1 does not mean that the PR gets rejected immediately. It means that it requires more work before having a second chance (well, in some cases it will mean a rejection depending of the reasons of the vote). And also keep in mind that a -1 vote can be changed later when the discussion on the PR continues