i don't think it's as complicated as it seems at the college level or below...
defense is reactionary- the squad may go out with one intention or set, just to realize it's no good and shift out of it.
on elite college teams and certainly in the pro's, the defense will 'draw' the offense by showing one look just to act like they know what the offense is doing shifting into another (smiling and taunting, pointing and threatening) and then getting the offensive play they actually want to begin with- this is knowing the audible tendencies of the opponent based on down, field position, clock, ect... O's counter by doing the opposite- marching to the line in one set with full intentions to shift into another--- making it 'not' an audible.
John Chavis, while DC at UT once in a blue moon was cut loose- and the defense would float like mad- no down linemen, lots of leap frogging linemen and LB pulling up and dropping back- it looked crazy, but it was to NOT show the O what set they were in. I wish they would have ran it more as it discombobulated the O more times than it didn't, and when it was all said and done, they were just in a 43. the players liked doing it though, it was plain to see.
but as far as method to madness, at the college level and below, i truly believe the players are picking up things they saw on film such as a slot showing intent to of picking the corner in a man-to-man and giving the LB heads up he's gonna hafta cover than- or seeing a OG ever so slightly shift his weight indicating he's going to pull or stunt and moving the DT six or eight inches inside or outside making the OG's job of picking up the block more difficult...
i like watching this cat and mouse game as much as watching elite players make the plays that make them elite..
it's been said a thousand times and i'm not the author, but- scheme puts the players in position, and the skill makes the tackle. for this reason it drives me nuts to see a defender either close on a player right before the snap or put too much gap by falling back, also right before the snap.... through the magic of DVR, you can rewind and watch either the DC or the position coach steam when they do that unsolicited. that's the O getting into the D's head, and it ain't an audible more than it is.
also you've got to consider ol' Madden's claim: Only an outmatched defense blitz's.... that's changed greatly this generation as defenses going on the offensive has become a lot more prevalent- but there is still a truth to it... defenses built around blitz packages as a matter of course are basically attempting to take ownership instead of be owned. they're more often hiding something than they are showing how good they are.
all that^ said, there is something else going on before the snap, and it's tied to directly to: players are actors. some of them are better than others at acting. even at HS level, the D players *again being reactionary watch their assignments and notice if the player is acting too 'nonchalant' or too interested, looks too long at the QB or exchanges a glance with a key blocker (if running)....
i have a love/hate thing going on with the advent of visors.
Corey Moore is a good if not one of the best examples... DE for VT around the turn of the century... the dudes eyes were HUGE and occupied at least 50% of the real estate behind that mask it seemed. He was also a helluva pass rusher and could cover, too... i asked him about that visor of his and he laughed and said it's responsible for at least half of his being in the right position... he said he'd approach the line and never shift his head from looking one direction while his eyes were looking the other- he'd give the QB or runner warm fuzzies he'd been fooled.. the funny part is, if he was wrong, he'd already be in the players head he was looking toward without hardly noticing him... he felt strongly that's why he seemed to always be in the play.
George Coontze, former GB linebacker and from up the road a bit from me used to be a customer of mine- he and i would BS from time to time about the game, and he'd bitch about players like emmitt smith wearing their helmets low across the brow and keeping their heads down taking position- and that it wasn't so much that he did it when he was going to run the ball by design (a 'tell' and giveaway) but that he wouldn't forget to do it when he didn't run the ball- he said half the players production was based on those seconds prior to snap and how well they 'sold' their part... emmitt excelled, and bought him a fraction of a second at least half the time, and that's all the time a player like emmitt needed.
like i said... actors.
and a recent case in point- the Bama QB who threw the OT streak TD to win the game.. he 'looked' the defender off the receiver for the separation he needed- he sold it well... Baker Mayfield is damn good at 'looking' players away. i could go on... i'll shut up now.