Very quick and dirty.
One of the most misunderstood aspects of J is that people think they don't change their decisions.
That is not the case at all, really.
It simply comes down to this: Do you need a decision up front (so you know what to do next) and then keep processing, while not really letting on you're processing? Or do you refuse to commit to anything while you ponder the situation?
I used to think J's made decisions quickly and then stick to them.
They do make them quickly.
But you can keep pushing on them, giving them new information, observe them seemingly deny all the new information...
... and then one day, poof, they just change their stance "out of the blue."
And pretend that they're sure about that one too.
Underlying principles:
1. J's want to take action -- it's more important to do something rather than nothing. Hence, they get closure so they can act and maintain some control.
2. P's want to be right -- it's more important to hold off on a decision that is uncertain and do nothing, rather than do something and have it be wrong. They don't want closure so they can remain flexible for the "best solution" and/or not be mired down.
Put another way, J's like to be more proactive, even the introverts (who just do it on a low-key scale). They gun for closure and make decisions and take action and shut off the information flow when necessary. P's like to be reactive and tailor their actions to the information flow coming in; whatever leaves them the most flexibility to respond is preferred.
Sorry, that was dirty but not quick.