The bigger difference between primary-trained skill and cross-trained skill, the bigger % of CT goes to cross-trained skill.
So let's say you have a player with 6 guard skills at 10 and 4 inside skills on 1 and you are training OD. If CT chooses an inside skill (save ID which is a secondary OD training), 85% of training will go to OD and 15% will go into that inside skill. However, if CT choosses a guard skill (in this case either JS, JR or PA), OD will train on 98% speed and 2% will go to that CT skill.
% numbers are from the top of my head of course and are there just to illustrate the point.