T' negative base drag (see "base drag") that can result when t' aft o' a movin' projectile is tapered such that t' airflow around t' projectile expandin' into t' space behind t' projectile presses against t' taper, providin' a small amount o' forward thrust. T' effect is similar t' squeezin' a watermelon seed betwixt your fingertips and sendin' it flying. It is also similar t' t' way a spike (or aerospike) nozzle provides thrust. Sometimes called t' "Pumpkin Seed Effect".