it locks your seatbelt in place which simulates a fixed racing harness... of course its no substitute for a full harness but at least it does a better job than a stock seatbelt which usually just flops around.
you can go the site and they have a full explanation on what it does.
i got it because a lot of track junkies use it and recommended it to me but i dont track that much so i am selling it.
Current: 2013 F10 M5 & 2013 GLK350
997.1 GT3RS (gone)
2009 WRX STI (gone)
e92 M3 (gone)