Here is a demonstration of light being eaten up by a Stoffen. In this case, we are using flash fill to reduce shadow intensity of shadows left by the sun. On the left, no flash. In the center, flash fired with only the inherent flash lens (clear), at the right a diffusive surface which is no larger than the flash lens (a Stoffen-like surface)
Notice that with a diffusive surface the shadow intensity is NOT reduced by the same degree as the native clear flash lens! Notice also that where a shadow is created by the flash (visible immediately adjacent and just to the left of the fruit picker, at red arrow), the diffusive surface has little to no effect
on the quality of the shadow penumbra (edge) --which is affected only by light source SIZE, which is no different because the diffusive surface is no larger than the native flash lens itself.
Now we have the Stoffen-sized diffusive surface vs. a 5"x7" size softbox over the flash head.
Notice the shadow edge characteristic is softened with the larger surface area (35 sq.in.) on the right, versus the original 3 sq.in. of the small diffusive surface.