Only players which received minutes on trained position during the week will receive the training. So if you played your Shooting guard on C for 30 minutes and you are training Inside defense for C, your shooting guard will receive 30 minutes worth of training.
Most important thing with training is you have to ensure full 48 minutes to your trainee. Every lost minute lost, because 24 minutes of training does not equal 50% of 48 minutes, but much less (loss is progressive).
So if you want to make your Center better at outside defense, you have to switch to Pressure training for PG and give your C 48 minutes of playing time on PG spot in that week.