Picture this.

It’s the third week of the college football season, and, in the latest chapter of a hot-running rivalry, Southern is shooting for an upset win over Jackson State. It’s late in the fourth quarter, and the Jaguars trail by four points.

Suddenly, wide receiver Jared Green catches a pass, turns on the jets and leaves the JSU defense behind.

Green is all alone, nothing but the goal line in sight. At the 15-yard line, he places one hand behind his helmet and high-steps into the end zone, ? la Deion Sanders. The fans at A.W. Mumford Stadium go nuts.

Then they see a yellow flag at the 15.

The referee calls an unsportsmanlike-conduct penalty - and it won’t be enforced on the next kickoff. Instead, it’s a 15-yard penalty from the 15, where Green’s showboating began. And the touchdown doesn’t count.

Then the fans really go nuts.

But because of the NCAA’s newly modified sportsmanship rule, the referee is correct.

“The points never go on the board,” said Harold Mitchell, a former Big Ten back judge and now the coordinator of football officials for the Southwestern Athletic Conference.

“It would be first-and-10 from the 30.”

Welcome to the brave new world of college football, where stricter celebration and taunting rules can result in game-changing penalties.

Obviously, the new rule has coaches on alert.

“We’ve talked about it with the players,” SU coach Stump Mitchell said. “You have to talk about it, because all of a sudden, you want guys to stop doing something that they’re accustomed to doing.”

Some of the players say they’ve gotten the message.

Green is among them. No high-stepping for him.

“I won’t do anything - other than run into the end zone, hand the ball to the ref and run to the sideline,” he said. “And then I might celebrate with the people on the sideline.”

Of course, the rules weren’t always so strict.

In 2009, for example, SU’s Byron Williams flipped into the end zone at the end of a 91-yard kickoff return against Alcorn State. Officials flagged him for unsportsmanlike conduct and assessed a 15-yard penalty on the next kickoff.

This season, the touchdown wouldn’t count. Instead, Southern would have a first-and-10 from the 18-yard line - some 15 yards from the 3, where Williams’ leap began.

Earlier this month, Williams smiled and shook his head, saying he understands that flips, dives and other acrobatics are off the table. Or else.

But the new celebration penalties can be just as crippling to defenses.

Let’s say it’s third-and-17, and a defensive end sacks the quarterback for a 10-yard loss, forcing a fourth-and-27 and a certain punt.

But hold everything. The defensive end stands up and does a Shawne Merriman-inspired “Lights Out” dance.

If that happens, flags will fly. The offense will get 15 yards and an automatic first down.

“You have to show that (NCAA) tape and talk about it constantly, because you don’t want to stop them on third down, or even fourth down - and all of a sudden, you give them a first down because of the penalty,” Stump Mitchell said.

“And the fans are going to be ticked off, saying, ?Hey, he didn’t do anything.’ Well, really, he didn’t. But as far as the NCAA is concerned, because of the rule change, he did.”

Green said the team had “movie night” earlier this week, watching the NCAA video of dos and don’ts.

“There might be a couple people who don’t feel it’s good,” Green said. “I won’t speak on how I feel about it. But what I will say is, we’re going to emphasize the part that’s even more fun, which is scoring and winning.”

The rule does not have etched-in-stone guidelines, Harold Mitchell said. In some cases, it’s a judgment call.

For instance: If a player simply raises the ball above his head before reaching the goal line, that’s probably OK. If, however, the player reaches backward with the ball, taunting a defender on his way to the end zone, that’s probably a flag.

But there could be a wrinkle. Harold Mitchell said some players might look like they’re not showboating at all - but if they’re cursing or talking trash, they’re in danger of getting a flag.

“If he is verbally baiting or taunting, then, yes, we’re probably going to get him,” Harold Mitchell said. “And I would hope our referee would indicate something like that when he makes the call, so the fans would know.”

Harold Mitchell said he suspects most officials would prefer that the new rules weren’t in place; that way, the officials aren’t drawing as much attention.

Nonetheless, the officials have to abide.

So, too, does everyone else.

“If that’s the rule, that’s the rule,” Green said. “And if you’re good enough to go to the NFL, then you’ll be able to do what you want.”

Fan Day, scrimmage

Southern worked out in shoulder pads and shorts Friday afternoon. ... As practice ended, RVs started pulling into the parking lot north of A.W. Mumford Stadium, already anticipating Saturday’s scrimmage and Fan Day events. Southern will hold its final preseason scrimmage at 11 a.m. Saturday inside the stadium. ... The season opener follows one week later, at 6 p.m. Saturday in Nashville, Tenn., against Tennessee State.