You start with a point pool (300 NG). The weapon takes a point, picks a combatant to assign it to (you or the enemy), picks a stat (boost/bonus/defense), and then decides if it will be a positive point or a negative point. The weapon is weighted to work in your favor so usually your stats will end up positive and their stats will end up negative, but the SC and NG versions have "better" weights so in addition to getting more points to assign there's a greater chance that the point will be positive for you or negative for the enemy.