There is documentation in the zip files.
I originally designed SdFat for recording audio and fast data logging. I added features similar to the POSIX real-time extensions for files.
Large contiguous files can be created quickly. There is a method to do raw writes to the SD using an efficient multi-block mode. When a file is closed, the unused space can be truncated.
This allows very low latency writes to high quality SD cards. The best cards are standard cards (2 GB or less) such as the SanDisk Extreme III. Manufacturers have stopped producing many of these consumer grade cards so I am now experimenting with industrial grade cards like those made by Swissbit.
The write time for a 512 byte block is about 820 usec with good cards.
I have had some luck with SDHC cards like the 4GB and 8 GB SanDisk Extreme cards but manufacturers assume devices that use these cards have lots of RAM for buffering and can tolerate an occasional long write latency.
Download the 100,000 sample per second logger for an example that uses of these features.