Reason you can do that is that exposure to air can help callous the wounded ends. Succulent cuttings need to be calloused before being put into soil or wound will rot. After a while your leaves will seek moisture and put out rootlets from the calloused ends. When get a few roots appearing and seeking soil, you can then tip the leaf and bury the root end down into soil and leave rest up. New leaves will appear and the original leaf will die.
