1. Trainers
batting or bowling net - slowest. If you give a trainee batting or bowling nets, you'll see about one consistency or concentration pop (depending on bowling or batting) in a season for a 17yo. Obviously, if you give them both batting and bowling, you'll see a pop in each.
batting/bowling and fielding/keeping - moderate. Adding a fielding or keeping net halves the time to pop. It's also better for your sanity, as trainees will pop in three skills with some frequency.
batting/bowling fielding and keeping - fastest. Shave another two weeks off by using both keeping and fielding at the same time. A player on batting, bowling, keeping and fielding nets pops all the god damn time, but of course keeping and bowling are mutually exclusive so some of the pops are just for show. All the same, good to see on a Monday morning.
2. Sports Psychologists
The SPs are what cause secondary skils to rise. The way in which these work is that they attend all unique nets which aren't stamina for each player.
Eg,
Batting [Don Bradman]
Batting [Don Bradman]
Batting [Don Bradman]
Fielding [Don Bradman]
The SPs will attend two of The Don's sessions, hence the configurations named in part 1. Like with nets, the first SP will do most of the work. The second will do less and so on, to the point that it is most accepted that 5 SPs should be on staff, 7 being the next most accepted number.
3. Ageing
Unlike primaries, secondaries become easier to learn as the player ages. However, the gradient is much flatter. Instead of training time doubling by the time a player reaches 24, it will take about 10 years for a player to halve his secondary training times. As secondaries initially take far less time to train, the effect is that it might be possible to train an extremely old player in just a couple of weeks.