For your life total to become a certain number, you gain or lose the corresponding amount of life. For example, if your life total is 4 when the ability resolves and your creatures have a total of 6 toughness, the ability will cause you to gain 2 life; alternatively, if your life total is 10 when the ability resolves, it will cause you to lose 4 life if you choose to change your life total. Other cards that interact with life gain or life loss will interact with this effect accordingly.