How it all works exactly, I'm not sure. All I know is that it doesn't work on the block level like most RAID's do, but rather on the file level. In other words, a layer is laid over the existing file system and FlexRaid takes over. Using this method it's able to pool the harddrives into one big volume (like unRaid does, but easier since you don't have to preclear the drives and you can also add drive that already contain data), and it also offers redundancy by use of a parity drive (also like unRaid does). The main difference here is that you can choose how much redundancy you want. UnRAID is only able to use one parity drive, hence only one drive may fail. With FlexRaid, the amount of parity drives is not limited. If you wanted, you could use 5 parity drives in your array, meaning 5 drives may fail before you're in trouble. (and even then, only the failed drives will have lost their data, the other drives in the array can still be read by simply putting them in a different pc)

Obviously, the stress here is on pooling and redundancy, but this doesn't offer any speed increased like normal RAID (except RAID1) or ZFS does. On the other hand, it sounds very safe, easy and convenient.