See thread here:
viewtopic.php?f=37&t=4308
Current system is that if a rookie averages 30 or more mpg, he can be trained 3 points in current rating only and cannot raise an athletic rating. Players who are not rookies get no benefit. New system allows players to benefit not just during their rookie season, but at any point in their rookie contract.
This rule would take effect beginning with minutes played in the 2016-2017 season (so players who just finished playing their rookie seasons would still be trained under the old rule).