At the end of the first perek of Massekhet Gittin we learned that a divorce cannot be granted after the husband has already died (see daf 13a-b). Therefore, if the husband orders a messenger to deliver a geṭ to his wife, should he die before the geṭ was successfully delivered, there will be no divorce (i.e., the woman will be a widow, not a divorcee).
The Mishna on our daf discusses cases where the husband makes the divorce conditional on his death. It is fairly obvious that if he uses an expression that indicates that the divorce will take effect when he dies, then the divorce will have no meaning. If, however, he says that he is giving her the divorce and that once he dies he wants the divorce to take effect retroactively from today, then the divorce will work, according to his instructions.
One question that is raised by Rabbeinu Tam in Tosafot is what the halakha will be in a case where a man gives a geṭ to his wife, saying that when he dies he would like it to take effect from today – and then he dies on that same day? Does “today” mean the end of the day and the geṭ effectively comes too late? Should we distinguish between a case where he said that the geṭ should take effect “from today,” and where he said that it should take effect “from now”?
Rabbeinu Tam writes that he cannot reach a conclusion with regard to this question. Tosafot quote Rabbeinu Elhanan who writes that this is a case where we must try and evaluate the thought process of the husband. It is clear that he is offering this divorce because he fears that he will die. If that is the case, we can conclude with certainty that the fear is an immediate one, and that his intention was to divorce her immediately, even if he were to die the same day.