Acharei Mos

The Posuk(18,5) says אשר יעשה אותם  האדם וחי בהם which man will carry out and live by them.  אונקלוס says that וחי בהם its referring to חיי עלמא which is Olam Habba.
The gemara in sanhedrin says that from the words וחי בהם we learn that if a life is at stake then all the mitzvos in the Torah can be transgressed to save that life. So we see that it's referring to life down here, not like אונקלוס who says its talking about olam habba. 
The Mishnah in Avos says Ben Azay says "one should run to an easy mitzva and run from sin, for one mitzva brings another and one sin brings another". רבי רפאל כץ מהמבורג asks, shouldn't it have said first to go away from sin & then to run to mitzvos? Like it says, סור מרע ועשה טוב go away from bad then do good.
The shulchan oruch says that פיקוח נפש pushes away shabbos even if you know the person could only live for a few moments. The מאירי  reasons that because even in a few short moments the person could gain mitzvos & good deeds, & with those, gain his place in the world to come. Like it says יש קונה עולמו בשעה אחת. 
This the שפתי רצון says, is what אונקלוס is saying. He explains to us how we see from וחי בהם why you're able to transgress the torah to save someone even if it's just momentarily, for in that moment one could be worthy of eternal life because  יש קונה עולמו בשעה אחת. There are those that acquire their world in one moment. I once heard a different interpretation of the word שעה - to turn. Like it says by kayin ואל מנחתו לא שעה, and to his offering He didn't turn. To use it in our verse יש קונה עולמו בשעה אחת, there are those that acquire their world with one turn.
Maybe that's why Ben Azay says to run first to the easy mitzva for all you have to do is make that one turn; make that one decision, and in one moment you can gain eternity!