The pick play is one of most effective and difficult-to-defend in football, often freeing up receivers for big gains or creating an extra bit of space in the crowded red zone.

How and when a penalty is called on the play is often is a point of contention for players and coaches.

"What you want to look for, is it truly a situation where the offensive player prohibits the defender from making a play?" NCAA coordinator of officials Rogers Redding said on Sunday. "It's got to be obvious and the rules even says, 'an obvious intent to impede.'"

A seemingly-obvious case came Saturday night, when an offensive pass interference penalty wiped out what appeared to be a last-second, game-winning touchdown by No. 5 Notre Dame against No. 2 Florida State.

Trailing by four with the ball on the 2-yard line, the Irish bunched up three receivers to the right side. Upon the snap, C.J. Prosise and William Fuller dove left and Corey Robinson slipped in behind them to the right, catching the ball in the end zone with 13 seconds left.

The Irish celebrated while Florida State's defenders threw their arms in the air, claiming they were illegally picked off.

The officials agreed with the Seminoles, calling a penalty after Prosise and Fuller made contact with the defensive players. Faced with a fourth-and-18 after the penalty, Notre Dame quarterback Everett Golson threw an interception in the back of the end zone, sealing Florida State's 31-27 victory.

Officials will usually allow hand-fighting between players and typically will not throw a flag if players' feet get tangled up. Even if there is contact, they often won't call the penalty unless the offensive player obviously caused the contact intentionally.

"What receivers are supposed to do is go in there and stop. If you do that, then the refs will let you do it," South Carolina coach Steve Spurrier said. "But it appeared the Notre Dame kid blocked him the whole way. We tell them to stop in the path of the other guy and they'll have to go around you. But it looked like he blocked his way off."

Irish coach Brian Kelly was still convinced his players did nothing wrong after watching the film on Sunday.

He said the play called for Prosise to get into the end zone and clear a little space for a quick turnaround pass. Because of the contact and how quickly the play developed, Prosise was not able to get turned around before the throw was made, according to Kelly. He also emphasized that Prosise did not go out of his way to impede the defenders.

"It's pretty apparent what happened on the play: Florida State blew the coverage and got rewarded for it," Kelly said.

The pick play, or rub route, can be an effective way to create space in the crowded confines when a team wants to pass close to the end zone.

Bunching receivers together on one side of the field makes it nearly impossible for defenders to jam them at the line scrimmage, providing extra room. The receivers then run crisscrossing routes, creating a "pick" on the defender trying to stay with the main receiver.

The design of the play is to force the defense to switch off receivers or to slow the primary defender, creating precious room for a completion.

According to NCAA rules, the crossing receiver cannot purposely impede the progress of the defensive player. In both the NFL and NCAA, blocking more than a yard downfield prior to a pass being thrown is considered pass interference.

The difficulty for officials is determining whether contact with a defensive player was intentional or not.

The inconsistency on the calls is what frustrates coaches the most.

Earlier in Saturday night's game, the Seminoles complained that they were illegally picked on a touchdown by the Irish. Later, with the game on the line, they did get the call, preserving their undefeated season.

"There are certain angles you can take to run routes that might cut a guy off at the pass and force him to go over the top or underneath you," Georgia coach Mark Richt said Sunday. "I think you can do that very legally, but you definitely can't just go down the field and start blocking people and throw the ball past the line of scrimmage."

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AP College Football Writer Ralph Russo in Tallahassee, Florida, and sports writers Pete Iacobelli in Columbia, South Carolina, and Charles Odum in Atlanta contributed to this story.