Every time you train something, players will increase their skills. We don't see a "pop" until they go past a whole number, but every training incrementally helps.
When you change training (either position or skill type), the skills do not revert back to where they were prior to training... they stay where they are after the training. [So, as you say, it is locked.] Say, for example, you train jump shot for 3 weeks and the is no "pop." The next time you train JS, there is a pretty good likelihood that a player will pop. [All of this depends on a number of factors, like age, soft skill cap, and trainer skill.]
It should also be noted that training in one skill primarily trains a skill... but also give you skill increases in other areas. For example, I trained JS for Guards this week. One of my guards popped only on JS. Another popped on JS and Handling. A third popped on JS, Handling and Driving.
Now, if I continued to train JS... I would not expect to see a triple pop on the 3rd guy for quite a while. However, the first guy might get a pop in a different area that was not JS next week.