From what I've been able to figure out, they pretty much get into our heads, and sometimes our bodies. Change things around somehow. Memories, skills, alliances, beliefs, and sometimes even our bodies. Keep us like that for a week or so, and then it's over and everyone reverts. Sometimes people remember what they did. A lot of times they don't.

Supposedly, it's because they're trying to figure out how to send everyone home, and whatever mumbo jumbo they're trying affects us in here. Not so sure I believe that, though.