For a series of events
P = 1... okay 2... now time for 3... and finally 4
J = 1,2,3,4, all planned, initiating sequence
Ps can know what they want to do, but often change their mind when it comes
I sometimes think of it as P makes a lot of smaller but spread out decisions (decide when I get to it) and J makes one plan and order to things, keeping a schedule or some kind of model of the events.
Also, if you're an ISTP you use
Introverted Thinking, yet if you're an ISTJ you use
Introverted Sensing.
From mypersonality.info:
Introverted thinking function allows a person to categorize and analyze data. It is the ability to identify inconsistencies, know how things work and problem-solve.
Introverted Sensing function allows a person to remember data in detail and be able to compare it to the current data. It is the ability to link present experiences to past experiences in search for a connection.