I've asked around and received so many different answers so please clear this up for an M6 guy.

 

The lock up refers to a sort of clutch inside the torque converter itself that when engages physically (through friction material) connects the input and output splines of the torque converter in a 1:1 ratio.

Basically making it like a heavy version of your clutch.

It is almost exclusivley used on new cars now because of mileage benifits on the highway, and the only reason not to get one is if you are a serious racecar, etc.


Chris