Additive Blending

When you select Additive Blending a tool reapplies its effect over an area that you have already applied an enhancement to.  If the option is left cleared the effect is only applied once.

White Pencil applied with 50% Opacity and no Additive Blending.

White Pencil applied with 50% Opacity and Additive Blending.

Notice how the color is applied over top again at each intersection.