Once upon a time there was a Pitcher for the Mariners, I can't remember his name now but for like 4 years he was telling batters when he was going to throw a certain pitch, the batters would just wait and take balls and or strikes waiting for the tell.
How did we find out?
Finally we made a trade for a player that disclosed it, he had stated that it's pretty well known around the league, why didn't the Mariners pick up on it, probably because they are looking at other teams and not their own.
Cable doesn't look at his line for tells, he looks for technique and missed assignments, he doesn't look with the eyes of a defender.
It would explain a lot about how quick lineman shoot gaps on certain plays, why we have contact in the backfield so much, just like when C Mike was giving away where he was running and when he was going to get the ball early his Rookie year, good defenders will look at eyes, foot placement, where your hands are on plays your in the pattern and those your not, if you have them on your hips lets say as a decoy on the line versus ready to hand fight and up when your going to be trying to get off the line.
Or how deep a back sets up on passing plays versus a running play and where his feet are, is he cheating with his feet towards the hole and or leaning a direction.
Never played a linemen, but I am sure there are tells that D lineman can read as well, spacing, hands, feet, how deep you set up etc.