After the teeth have been whitened, they are extremely porous and like little sponges. So as soon as you go home and drink a coffee (or enjoy anything dark/acidic), those stains are going to want to stick back onto the teeth in the little pores that have just been whitened. Stain Shields help prolong results, rehydrate enamel and ease sensitivity by filling these pores with potassium nitrate, making them glossy & hydrated so that the stains just bounce off the teeth instead of sticking to them.
 
Stain Shields Instructions: Just before bed, brush, floss and rinse your teeth. Twist up the bottom of the pen until a small amount of gel comes out and brush onto the fronts of your teeth. Do this every night until it is empty. No Eating, drinking, spitting or rinsing.