Not flaming; just curious: has this actually been tested? The reason I ask is because that 4 GB of write cache is on the SSD, no? So how would that make any writing to the FD faster than a pure SSD?
Because of the way SSD drives work.
An SSD drive can read 4KB blocks very quickly, it can write 4KB blocks into an empty block quickly, and it can erase pages of typically 128 KB. If you have a page with 32 blocks and want to overwrite a block, the SSD drive can't do it. It's just physically impossible. An SSD drive can't write to space that already contains data. What it would have to do is read the other 31 blocks, erase the page, write the 31 old blocks and the new one. (Of course, the firmware on an SSD drive will do whatever it can to avoid the situation; old drives had massive slowdowns on writes because of that).
Fusion has 4GB of already erased space ready. So if you write many small blocks, they can be written into that erased space at full speed. Then when the drive is not busy, it can do the hard work without you waiting for it.