I can't find the quote, so hopefully someone comes along and posts it here, but the basic idea was to make more well-rounded players and to limit the amount of high salary, monoskilled bigs with terrible subs. For whatever a player has been trained in, 90% of the training goes to that skill, and the other 10% goes to another skill. For example, if Player A is trained in PA, 90% of the training goes to PA, and the other 10% goes to something else. Or if Player B is trained in Stamina or FTs, they may get a pop in a different skill because the 10% went to that skill. As far as I know, it can come from training anything.