It's called omni-bounce from Sto-fen. It takes away about two stops of light. If you use TTL, you don't have to make any adjustments, just make sure you get close enough to your subject, because you can't use your maximum distance as displayed on the flash. Each flash has also a minimum distance. Using the diffuser, you can shorten that distance without risking to overexposure your subject. In macro, when you get really close, it is good to get even lighting and less harsh shadows.
>Each flash has also a >minimum distance. Using the diffuser, >you can shorten that distance >without risking to overexposure your >subject.
I don't know where that "shortest" distance come from, but I have take photos with my F90x and SB-26 from 5cm. even flash says that minimum distance is 60cm. It's TTL so it should not have minimum distance! (I think)
I have been using an Omni Bounce with a Vivitar 283 for years. I have had the flash for 22 years and the Omni Bounce for aout 3 years. Never had any problem. Matter fact it was the best thing I could purchased for that 283. Can't wait until the one for my SB-28 comes in.