They're to run with the receiver, not look back for the ball until they read the WRs eyes and see him looking back for it. If the DBs run while looking back for the ball, you'll see the burn percentages skyrocket.
Some DBs (Eddie Jackson on this team) do a much better job of correctly reading the receiver's eyes.
Heard this same thing, phrased as only he could phrase it, direct from Joe Kines.
He said if the DB was "in phase" (Kines' words, not mine), they're coached to look back for the ball. If they're not in phase, even if it's by a quarter of a step, they're coached to knock the ball away as the receiver tries to catch it. Or, if the receiver makes the catch, minimize the damage by making the tackle immediately.
In phase was defined as being on the receiver, step for step, and in position between the QB and the receiver -- what most of we non-professionals would simply call "blanket coverage."
The reason was just as Jess stated. If the DB isn't in phase, and still looks back for the ball, the distance between the DB and the receiver inevitably increases, and the possibility of a long play increases exponentially.
Kines also said this was one of the hardest points to get across to a young DB, especially a highly recruited one (which would be all of ours under Saban). Too often, they can't admit, even to themselves, that they're beat, even by a minuscule amount.
It's just one of the things that lengthens the learning curve that all freshmen go through.