It's been said that the true Raiders hat doesn't show up untill Indy swings over the pit. The hat can barely be seen in the dark first shots in the Peruvian cave, but there often seems to be taper and an unremarkable brim. I found these incredible frames in the tarantula sequence, and knew that the ribbon must have been overtightened to bunch the felt like that.
After this shot, the hat was turned, the front pinch repositioned, producing (completeing) the true Raiders hat, but the spot where the felt was bunched remained, leaving the channel under the ribbon in the middle of the right front dent.
....The ribbon was mostly overtightened on the top, but also somewhat on the bottom. Look at the angle of the ribs where the ribbon meets the bow on my tightened ribbon.
Also notice that the corner of the ribbon pops out under the bow. You can't see the angle of the ribbs on the Raiders ribbon, so I can't say if it was tightened at the front or back of the bow, but it looks like the ribbon is popping out under the bow there too.
At the front pinch is where a too tight ribbon would produce bunching. The felt can actually fold here.
Then I put the overtightened ribbon on and there's the tarantula hat.
Now I roll the front pinch to the left, and that's the turned Raiders hat with the channel under the ribbon in the middle of the front dent.
......Besides getting the channel under the ribbon, without stuffing toilet paper in the swetband to get it, I'm pleased to see the bow angleing inward at the top, creating a flat spot on the side of the crown above it...Raiders features I wasn't getting before. .. I believe the Raiders hat had 3 front pinches. The normal dead center one, the final one 1/2 of an inch to it's left, and one a half inch to it's right- this last one is where the channel under the ribbon formed, and I think you can see the whiteish line that runs up the front dent above it where the pinch used to be ..so the felt was already weakened there, letting the channel form easily.
A ribbon mainly overtightened at the top should give you less taper, more backtilt, mushrooming and other Raiders things.
Thanks again! 
Regards. Michaelson
--You could always get a larger hat and stuff the front. Sounds like a plan to me, Jersey 
If Ford wasn't in the unusual habbit front pinching way down into the ribbon, there probably wouldn't have been a "Lever" for the ribbon to push on and cause the bunching in the tarantula hat. If the hat didn't have a short sweatband, that didn't hinder deep pinching, the lever might not be there either. Without 3 front pinches it might not of happened. I've changed the title of my Raiders Turn page to "Raiders Turn, Ribbon Tightening and Sweatband Trimming". Here's a frame from the film when Forrestal shows up. I'll wildly speculate the hat is turned here, but the ribbon untightened- the channel isn't there. The bump in the right dent I'm guessing is the third front pinch where the channel eventually is, popping back out for some reason.? who knows...
Maybe when the sweatband on Indy's hat was sewn in something happened? Or maybe they put one in on purpose like I did?... Here's the hat with the pleat, and an even tighter ribbon. 
I wasn't trying to get this, but I just started noticeing...my top dent is now deeper, well it was touching the top of my head before, so the bumps are taller, I guess, or it just has steeper sides or something. 
..........Whether or not this ever happened, this is a cool way to make a loose hat tighter. Instead of stuffing something in the sweatband, this actually recduces the circumfrence of the crown!....Maybe the Raiders hat has two of these......you know that "front pinch" under the ribbon that's always there?