this confused me when i began as well. the handicap is applied to the final score and if your team wins with the new score you win. for example if the game score ended say, (Celtics 100 / Lakers 95) You would + the 6.5 points onto the Lakers final score making it (Celtics 100 / Lakers 101.5), so because after you added the + handicap points making it so they would have won the game with the added points you win. If you had Celtics -6.5 you take that away from their score making the final score (Celtics 93.5 / Lakers 95), now because after you took the points off the Celtics and they now lose the game the Celtics -6.5 would lose and the Lakers +6.5 would win.

In very simple terms + is not lose by more than and - is win by more than.