LOS ANGELES - Halftime lockerrooms are sacrosanct. Only the privileged few are allowed entry. They’re like the situation room at the White House, only with tighter security.

What transpires largely depends on the team and the circumstances. Players get yelled at. Players yell at other players. Athletic directors yell at players, especially if they’re behind in the game and a lot of jack is at stake. Injuries are treated. Egos are massaged and bruised. An occasional meathead will bash his head against his locker for motivational purposes. Pain relievers are handed out.

At USC, the halftime process is similar to watching Cirque de Soleil. Afterward, observers turn to each other and exclaim, “How did they do that?”

Head coach Pete Carroll and offensive coordinator Norm Chow usually sit in a laboratory, rubbing their hands together with diabolical glee, as they concoct potions, curses, formulas and schemes designed to vanquish the opponent. They repeat a few chants, burn some incense, then burn the other team.

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You can count on it. It’s one of the reasons the Trojans won a share of last year’s national championship, are currently ranked No. 1 in the country in both polls, and have won 13 games in a row entering Saturday’s showdown at the Coliseum with the last team to defeat them, seventh-ranked Cal.

Making adjustments at halftime is nothing new in football. But Carroll and Chow have turned it into a near-exact science. The most recent example came last week at Stanford, when USC gave up 291 yards to the Cardinal in the first half but only 36 in the second. The result was a 31-28 victory.

It’s been that way since shortly after Carroll took over the program in December of 2000. But the flip side of that is a mystifying propensity to create the need for halftime adjustments. Often these Trojans start out slow, feeling out the opposition like a fighter does to another pug, then cranks up the attack in the second half.

Even though it looks like it’s by design, Carroll insists it’s not.

“Everybody’s concerned about this, but I’m probably the least concerned,” Carroll said about his team’s slow break from the gate. “I know you can’t win the game in the first quarter, no matter how hard you try. And you can’t lose it, either, unless you give up.

“Most of the time, from the first half to the second half, things that are adjusted are basically going to the rest of what you had prepared. It’s not like you throw away your game plan and start over again. It’s calling on things you didn’t get to yet.”

Still, it’s hard to deny that the Trojans often appear to hit the snooze button before finally facing the day. In the opener on the road against Virginia Tech, the Hokies jumped out to a 10-7 halftime lead, largely with the help of quarterback Bryan Randall’s rushing. But after some halftime tinkering, USC smothered Randall, gave up only three more points, scored on two long passes from Matt Leinart to Reggie Bush, and came out with a 24-13 triumph.

The Trojans’ second effort was a cruise-control special over Colorado State, 49-0. But the following week at BYU, the top-ranked Trojans again sleepwalked early before scoring three times in the final 8:30 of the first half to open the game up. Adjustments at halftime only made the rout worse for BYU.

Then came Stanford on September 25. That rousing display not only serves as the latest example of the halftime magic show conjured by Carroll & Chow, it also was a brisk smack in the face for the Trojans, who had become somewhat complacent and in need of an opponent tough enough to open the spigot for their competitive juices.

“These are the kinds of games we love, that we miss,” super soph Bush said afterward.

Still, there are those in Trojan Nation who wish the halftime adjustments and the wake-up calls weren’t necessary. A good, old-fashioned, wire-to-wire trouncing now and then tends to make the faithful less apoplectic.

“The part about not starting fast, I think it keeps people uncomfortable longer and that’s why they keep asking the questions,” Carroll observed. “They’ll just have to tough it out with us.”

SECOND-HALF SURGES USC is on a 24-1 run in large part because of their strong play after halftime Date Opponent 1st half total 2nd-half total 9/25/04 at Stanford Down 11 Up 14 9/18/04 at BYU Up 18 Up 14 9/11/04 vs. CSU Up 28 Up 21 8/28/04 at Va. Tech Down 3 Up 14 1/1/04 Mich. (Rose) Up 14 Even 12/6/03 vs. Ore. St. Up 14 Up 10 11/22/03 vs. UCLA Up 31 Up 8 11/15/03 at Arizona Up 35 Up 10 11/1/03 vs. WSU Up 5 Up 22 10/25/03 at Wash. Up 6 Up 14 10/18/03 at ND Up 14 Up 17 10/11/03 vs. Stanford Up 27 Down 4 10/4/03 at ASU Even Up 20 9/27//03 at Cal Down 7 Up 11* 9/13/03 vs. Hawaii Up 25 Up 4 9/6/03 vs. BYU Up 16 Up 1 8/30/03 at Auburn Up 10 Up 13 1/2/03 Iowa (Orange) Even Up 21 11/30/02 vs. ND Up 4 Up 27 11/23/02 at UCLA Up 14 Up 10 11/16/02 vs. ASU Up 10 Up 11 11/9/02 at Stanford Up 11 Up 21 10/26/02 at Oregon Down 5 Up 16 10/19/02 vs. Wash. Up 10 Up 10 10/12/02 vs. Cal Down 4 Up 6 * Includes overtime

“That’s not what we try want to do,” safety Jason Leach added. “We come out and try to play hard from the very first play. It’s not our plan to come out like it’s a boxing match and feel them out.”

Whether it’s intentional or not, it happens. And football wisdom suggests it had better not happen this Saturday when the Golden Bears prowl the Coliseum.

The last time USC lost was on September 27, 2003, in Berkeley, a triple-overtime 34-31  spine-tingler that ended a Trojan victory streak at 11 games. What is often forgotten about that contest is that the Trojans — surprise! — started out like chilled molasses. They were down, 21-7, before adjusting, revving up and rallying in the second half.

In the weeks following, Carroll would kick himself publicly. The Trojans had a bye week before that Cal game, and instead of keeping his starters sharp in practice, he gave extra reps to younger players. The regulars, Carroll lamented, became stale, resulting in a desultory beginning, which eventually led to USC’s only setback in an otherwise successful season.

Carroll backtracked this week, though, denying that approach in practice contributed to the slow start last year against Cal, and stating that this year — which also features a bye week before the clash with the Bears — would be no different in terms of preparation.

It may be gamesmanship by Carroll. It might be his way of lying in wait. He could be trying to lull his counterpart, Cal head coach Jeff Tedford, into expecting a tentative first half from USC and then whammo!, an ambush.

Among USC fans, that would be an extremely welcome change of pace.

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