I used it on my first playthrough(I'm a cheater I know ) For those that don't know it, here's how it works.

1. enter the formation screen and select a job that has a list of abilities that you can scroll up or down. make sure you have enough JP to learn an ability.

2. select the ability. it will ask wether or not you want to learn the ability. DO NOT CHOOSE YES YET!!!

3. While still in the process of purchasing the ability, hold the square button, and press up or down. what's this? it seems like the cursor is pointing to a different ability! If it is pointing to an ability that you can't learn yet, or one that you've already learned, choose yes. 3 different things may happen. a: you will lose a little JP. b: you will gain a little JP. c: your JP will go to zero.
ignore JP loss or JP Gain, but if it goes to Zero, exit the screen, and re enter it. you will have 9999 JP for that job!! this doesn't work for some jobs, because they only have one page of action abilities.


Also keep in mind that this cheat works for every job except, Squire, Knight, Thief, Archer, Monk, Bard/Dancer and Calculator.