For your life total to become half your starting life total (normally 10, half of 20), you gain or lose the appropriate amount of life. For example, if your life total is 4 when Torgaar's ability resolves targeting you, it will cause you to gain 6 life; alternatively, if your life total is 25 when it resolves, it will cause you to lose 15 life. Other cards that interact with life gain or life loss will interact with this effect accordingly.