From what I've been taught, there were traditionally, TWO ways that applications were taught:
1. If you were the heir to the system, the number one student or the son/daughter of the founder, you were force fed the applications and principles like a French goose being prepared to become foi gras.
2. If you were one of the regular students, sensei would throw you an occasional principle and you were supposed to figure it out for yourself. Lots of "monkey see, monkey do" going on here.
That said, over the last 40 years or so, I've picked up the following "bones" from various teachers:
a. Every time your arms cross in kata, there is a catch/trap/grab occuring.
b. every time you change directions in kata, there is a potential throw.
c. always look for the disruption of the oponents balance in a kata move.
d. there are NO superflous moves in kata. This is one place where John Sells and I digress. He thinks there are moves that are just for show. I can almost always find a way to make them applicable to fighting.
e. if a move ONLY works against an attack from one hand (i.e. a right hand punch) but won't work against an attack from the other side/hand, it isn't a valid application. True applications work against EITHER side attacks.
f. There should be a minimum of THREE, most often FIVE applications for each techinque in kata, generally:
-Disrupt the attack
-Grapple/Tie up
-Throw Take Down
with
Disable, i.e. break, dislocate, damage as the theme and
Destroy: kill, render unconscious or unable to continue
as the final ideas.
It just really isn't that simple. I have heard the "minimum 3, most often five applications" statement. I can think of a number of kata where it would not apply. I believe it to be modern "reverse engineeering." It was never mentioned to me by any of the three Okinawan/Japanese instructors with whom I have trained. Also, "every time you turn there is a potential throw" seems to be trying to force applications into a kata. Just because you or I can find an application for a move doesn't mean that was the originator's intent. Statements like these muddy the water too much and try to force kata into a framework that may not be there.