The negative base drag (see "base drag") that can result when the rear of a moving projectile is tapered such that the airflow around the projectile expanding into the space behind the projectile presses against the taper, providing a small amount of forward thrust. The effect is similar to squeezing a watermelon seed between your fingertips and sending it flying. It is also similar to the way a spike (or aerospike) nozzle provides thrust. Sometimes called the "Pumpkin Seed Effect".