As you can see, the flash is pretty much buried inside to the cone formed by the umbrella. This minimizes the amount of light spill you would get with an open umbrella but not as much as a conventional softbox,
This shot gives you a subject's eye view of the ISB. The flash is a Lumopro 180, which was one of the first to have a 1/4 x 20 threaded insert to facilitate near-axis mounting when used with an umbrella. Because of the "deep within" positioning of the flash, the surface area of the flash is both large, and contained. The photo clearly shows the borders of the light, and can be adjusted by choking up on, or extending, the umbrella shaft. I used a bounced flash to provide some fill and to trigger the optically triggered Lumopro.
I made this second shot without the fill light. You can easily see the disk of light created by the umbrella, which is probably equivalent to what you'd get from a 24" softbox. But unlike the softbox, the light "fades" as it moves towards the edges, so you won't get a clean edge that you can effectively feather. This is a pretty subtle thing. something I haven't experimented with in the field, primarily because one doesn't have much time to make those subtle, but necessary, adjustments to place this "twilight edge" where it could be used to its best advantage.
One small problem with this setup can be seen in the catchlight. This highly magnified selfie shows and exact reproduction of the umbrella and the silhouettes of the flash and the light stand in my left eye. And while the need for such a detailed closeup is seldom required or justified, it is there, just in case it matters to you or your photograph.
I will be doing a lot of experimenting with this improvised umbrella in the future, when time permits.