In ANH When obi wan dies he disapears because he distracts vader so luke can be safe, so he saved luke. In ESBYoda dies and disappears by training luke and giving him advise, so yoda also saved luke from being killed in a duel. In ROTJ when palpatine is nealry about to kill luke vader/anakin kills palpatine, Saving luke. So the jedi disappering trick is to do with luke, If you save him or help him and your a jedi, you will be one with the force a SPIRIT.