The conjurer quest line explains it pretty well. I haven't even got a level 1 BLM or ARC to say whether theirs has any but basically the conjurer calls upon nature's energy and redirects it. There is a girl who refuses to do this but can still cast spell using her own life force. Bad things happen I don't want to spoil it but conjurers merely need the will to use but use the energy from nature basically. Much like cars. Don't run cars on lifeforce.