I think Slohand translated the magic part of the k key perfectly here.
When i was testing the 38 it seemed once I hit the K key and held it I was able to release almost all pressure from the stick and it did a perfect loop a lot faster and tighter than I could have manually. Full K key is like temporarily going on computer generated autopilot in that situation.....and when your done with it just turn combat trim back on to acquire target or find the line you want to take next.
The slower you go the less affective it seemed though..... but I was able to find a few instances where the K key and flaps worked well together pulling the nose up quickly but very stable w/o any buffeting.
Last thing.
I was able to climb to 8k, pull up, hit the K key full, dump a few notches of flaps, cut throt a bit and do perfectly continuous loops over and over again without touching the stick. It just went round and round. In fact i got tired of waiting to see how long I could leave it like that without crashing and walked away for.....? ? ? not sure exactly, maybe 5-6 min only to come back and still see it looping around. It was a little crooked by that time and i lost some alt but it was still going round and round all on it's own. Try it 
Look mate I'm no expert so I can't explain technically as to what's going on here. I can only tell you my observations.
Lets deal with everything you've been posting in one foul swoop. Firstly take your P38 up again and press Alt R... record film. As you climb out press the page up page down keys. Press the arrow keys left right up and down.
Lastly press the down arrow so as to sit the pilot way back in the seat. This is most important for you to see what I'm about to explain.
Now do your "F" key trim up experiment but observe the yoke. Does the yoke not "pull back"? If the yoke is pulled back then the plane will do a loop....correct? If the yoke is kept pulled back consistently is the plain going to keep looping?
OK...now! With the arrow keys and the use of Z & ]]]] for zoom take a close look at the P38's trim setting. Right of the yoke on the dash. They are balanced under combat trim.....correct? Now trim up "F" and observer the trim.
Watch your film and you'll perhaps see some out of plane head views although in the plane your head never once left the confines of the cockpit.......It's the film viewer man.
Here's a link to dog fighter. Fast forward to 2 minutes 30 seconds.
http://www.youtube.com/watch?v=ITRLk9b9AcY&feature=PlayList&p=339B7D26B33FA7BA&index=0&playnext=1This guy pulled out by pulling hard on the yoke. Note the comment about denser air. Although he never commented as to what he was
focusing on inside the cockpit I'll wager it wasn't full steam ahead. That throttle would have been slammed shut for starters. As I said I'm no expert so to comment about whacking the trim wheel to max would be nothing more than sheer speculation.
For a contrast take a F4u or Spitfire up. Using keys observe the trim settings. There not balanced like the P38 are they. That's cause the plane is trimmed correctly for torq and the like. Balance those trims out and repeat your experiment. They loop as the P38 does cause the stick which you can't observe in the F4's is pulled back. You can see the spits stick pull back when you trim up.
It's OK to come here and make comments but if your not going to fully think about it and explore other possibilities your posting nothing less than:-
DISINFOSorry an all that but stuff like this should be talked about correctly before another AH urban myth is born. Unfortunately I don't have the "knowledge" to comment as to what actually is happening but it makes perfect sense to me that if the stick is kept back in the saddle nothing other than a loop will occur. Having observed the movement of the stick when trimmed my question to real pilots is this.....
Would that happen in a real plane and if not what does "technically" happen. Is it the ele's being set to maximum deflection as though the stick is pulled all the way back?