It add a fixed damage bonus to your offhand weapon.For example,without SS,your offhand max hit-152 min hit-14.After trained SS,your max hit increase to 160 min hit 22.So thats 8 more damage everytime you swing your 2nd weapon.Not a bad deal I would say,add to those AA nowaday(Accuracy,Ferocity etc) this will add 10-20 DPS directly to your damage output.A must have !!