If it's a draw that bet (of portion of the bet) is void. Same for -1, if that team wins by only 1 goal the bet will be void, for +1 the team can lose by 1 goal and your bet will be void etc.
Basically you have to get above 0, but unlike a normal handicap if you actually land on 0 the bet will be void, it just gives you that extra security.
That was long winded as fuck tbh and probably made nee sence.