To put it simple it's a crank pulley filled with hydraulic fluid that dampens the vibrations to the crank and rotating assembly including clutch. So when you start changing weights and going high HP you need it if you dont want to rattle your fillings out.I have seen a lot of guys on the MK3 forums talking about the fluidampr, the comment i always see is "vibrations rob horsepower and kill engines" but i have never actually understood what this kit does, they aint cheap that is for sure.