CPU calculates A+B=C for the graphics card, then sends C to the northbridge, who then sends it to RAM, who then sends it back to the northbridge, who then sends it to the graphics card.
Fast-Write gets rid of going to the RAM to make things quicker.
CPU calculates A+B=C for the graphics card, then sends C to the northbridge, who then sends it to the graphics card.
Latency is reduced, thereby making it quicker. However, there is a problem, performance gain is little and in certain situations performance may lag. Also, stability is lost. If you have no problems though, its a good idea to leave it on, unless you do have problems, which in this case, you would turn it off for troubleshooting.