Well, canon explicitly states that she uses it to "go unnoticed" in her official profile (that's how she got to the Moriya shrine without any of the Tengu knowing about it), and then there's all her "lolpsychology"-themed spell cards. That's it for canon, but you can extrapolate based on that; presumably, she's going unnoticed by moving your awareness of her into your subconscious, so that you "forget" you're looking at her. I'm guessing she can also go the reverse -- if there's something you're only subconsciously aware of, she could see it the way Satori could see your conscious thoughts, and use her power to make you consciously aware of it. In one of my fanfics, Satori realizes that something's amiss, but can't quite put her finger on it; Koishi sees what Satori "noticed" and moves it into her conscious mind. In another story -- this is on shakier ground, more of a "hey, that's doing stuff to the subconscious, right?" -- she uses her power to suppress Flandre's destructive urges, sort-of-hypnotizing her and making her docile. And, much like real hypnosis, it ceased to work when she hit something Flandre really didn't want ...