In general its to speed up the process of getting everyone weighed-in, as well as, it give them move areas to host the weight-ins since you don't have to be as discrete.
From what I am told from coaches it is 4 pounds over normal scratch weight for weigh-ins on just Saturday before the tournament. So 120 needs to be under 124.0.
Then the additional 2 pounds, 6 total pounds over scratch weight, for weigh-outs on Saturday night (or if you miss it 2nd chance weigh-in Sunday morning). So 120 needs to be under 126.0.
But, I hav not read this in a official statement from the tournament coordinators.