Can someone explain "bite inhibition" as it relates to leaving puppies together beyond 6 weeks?

This is just what I have always heard,although when I was a kid,we always got pups at 6 weeks and they all turned out to be very nice,even -tempered dogs. A friend of mine has a Labrador which is used as a breeding ***** for a seeing-eye school. She had a litter of 7 and was separated from them at 6 weeks and she was taken back home to her owner just the other day.So I guess if they separate pups which are going to be used as guide dogs at 6 weeks,they must know what they are doing.