From my vast storehouse of Asian magical lore gained from watching a lot of anime, so therefore we know it's totally accurate, those pieces of paper are probably magical seals. I don't know if they're Japanese or Chinese, but I've seen a lot of magicians slapping rectangular pieces of paper on stuff with dramatic moves and music.

A bit of a poke 'round Google turns up evidence that it might be more of a Chinese thing - Daoist in fact: here (http://www.logoi.com/notes/seal_magic.html) and here (http://www.eng.taoism.org.hk/religious-activities&rituals/rituals/pg4-6-10-2.asp).