There are a many of threads dedicated to using molasses on this site. From what I understood, if you are going to use molasses then it should be done early in the flowering process. I believe it feeds the enzymes in the dirt which in turn makes it easier for the plant to suck up the nutrients and food... overtime. There is also some that say that it helps to bulk up the bud. This was quite a while ago when I did all the research on molasses, but search the tags like Rasclot said.
From Cloud Nine,
Happy Growing