It's one of the most difficult judgment calls in all of sports.
Pass interference is a bang-bang play in which incidental contact is common, and the referee needs to keep track of two players' arms, legs, bodies and eyes.
Even when it seems obvious, sometimes it's not, as the Buffalo Bills found out last season in New England. A blown pass interference call against the Bills at the end of that game gave the Patriots the victory.
The NFL is trying to improve its pass interference calls this year.
In its season-preview press release, the league says pass interference will be a major point of emphasis among its officiating crews this season. Although there are no actual rules changes on interference this year, both offensive and defensive pass interference rules have been clarified so that they can be more easily understood by coaches, players and officials.
Last year, the description of pass interference took up 29 lines in the NFL rule book. This year it takes up 61 lines.
The Bills' defensive backs say they are clear on the way the rules now are written. But that doesn't mean they think the calls will be easy.
"The big problems with pass interference (for a defender) are grabbing and holding, not turning your head when the ball is coming and cutting the receivers off," said cornerback Ken Irvin. "Those are the basics I focus on. . . . But it's still a judgment call. Being a defensive back, yeah, it's definitely one of the toughest calls to make in the game."
"I'm pretty much under the impression that any kind of contact downfield with the receiver and you're going to get a pass interference or an illegal-use-of-hands call," said safety Henry Jones. "So I try not to contact them, and if I do it's got to be subtle or with my head looking back to the ball."
Contact beyond the line of scrimmage -- the defensive back is allowed to chuck a receiver within 5 yards of the line -- became a subject of controversy last year. Several NFL coaches complained it wasn't called consistently.
Bills defensive backs coach Bill Bradley said Miami and the New York Jets were two teams that succeeded in using more aggressive chucking tactics last year.
"They'd get somebody and almost tackle 'em, but if the ball was away (not thrown to that receiver) they never threw the flag," Bradley said. "The thing is, everybody needs to know, not just this team or that team. . . . Hand-chucking after 5 yards was not totally understood last year around the league."
Bradley said referees will allow some contact with the hands a few yards beyond the 5-yard limit if the ball is not in the air.
"If they start chucking them with their hands within 5 yards, they can still fight 'em a little bit as long as the ball is not thrown."
That, however, is risky, since it's hard for a cornerback to be chucking and watch if the ball is in the air.
New lines in the rule book more clearly describe legal incidental contact. That includes:
Laying a hand on a receiver that does not restrict the receiver in an attempt to make a play on the ball.
Contact by a defender who has gained position on a receiver in an attempt to catch the ball.
Incidental contact by a defender's hands, arms or body when both players are competing for the ball, or neither player is looking for the ball.
Creating "points of emphasis" in the rule book is an annual occurrence in the NFL.
"Last year they implemented the rule that if a defensive back ran at an angle and cut the receiver off, it was pass interference," Bradley said. "Two years ago, if a guy ran a stop and go, you could run and cut his angle off and make him change direction or stop. You can't do that anymore.
"One rule we got called on a time or two early last year," Bradley said, "was any time a guy breaks a route away from you and they see the jersey pulled and it affects the receiver's movement, the flag's going to come out."
"I understand the rules," said free safety Kurt Schulz. "But it's still so subjective. It seems to me, you're at their mercy."