The mastering limiter I am trying has a look ahead function. What is this function for?

Anybody know what the sonic differences are (if any). I know the NTP 179-170 has some more features, but I was wondering since everybody seems to have the older 179-120.

I recently added a SonicStudio HD system to our mastering room and I am seeking some suggestions for a brickwall limiter. I've currently got an API 2500 compressor which offers some nice old school character but I really need a decent sounding limiter. Unfortunately the local shops are no help since the salesmen typically don't use the gear they're selling.