Good question....A lot of the MN12 T-bird/Cougar/MK8 guys have driveline vibrations. This shaft allows for the rev and speed to go much higher without this vibration. Dennis Reinhart is selling them now. He has an 11 sec. MK8. Previously the only option was a 93 one piece MK8 driveshaft or a custom made shaft. I am no expert on this so keep that in mind!
Here's a link to his site.
http://www.reinhartautomotive.com/