Fit the decorating bag with tip 21 and fill 1/2 full with medium consistency icing. Hold the decorating bag 45° angle at 6:00; the tip should be slightly above the surface.
Squeeze bag to form a shell. As you begin to form a shell, squeeze hard, letting the icing fan out. To form curve, lift tip up and over the shell as you move tip from 9:00 (right or left handed) to 12:00 to 6:00. Relax pressure, lower tip. Pull tip straight towards yourself at 6:00 to form tail.
Repeat with another shell, curving from 3:00 (right-handed or left-handed) to 12:00 to 6:00.
To continue the reverse shell border, pipe a chain of swirling reverse shells, with the fan end of each new shell covering the tail of the previous shell. If you are making the border on a round cake, turn the cake as you go so that the back of the bag is at 6:00 and you are working towards yourself.