The idea took hold in 2013 at Arizona State. Why wave your arms to distract opponents at the free throw line, students wondered, when one of them could leap from a cardboard box at the right moment? From there it was a small jump to putting up a curtain, parting it at a crucial moment to reveal a Speedo-clad Michael Phelps and giving way to all sorts of outlandish efforts at distraction and intimidation.

This season, members of the Oakland University swimming and diving team, not content with foot-stomping, swaying, chanting and cardboard box-jumping, got into the act. A few weeks ago, video of the aquatic Golden Grizzlies gleefully shaving each others’ heads during free throws was shared nationally. From there, they took it to the team’s early Horizon League games played on their home court, rising to the postseason demand for more.

They waxed each other’s arms, legs and chests and gave Ian Allen an OU faux tattoo with a Sharpie.

There was a milk-guzzling contest during timeouts and halftime, too. “I was tatted up, waxed and full of milk,” swimmer Ian Allen proudly said of his commitment to a bit that grew after originating in the swimmers’ locker room, which they’d turned into a barber shop when most of them decided to shave their heads last month.

Allen, however, was reluctant and kept putting off the buzz cut — until swimmers decided they wanted matching hair “styles.” Allen agreed to shave his head before the basketball game against Horizon foe Cleveland State, and a teammate suggested he do it during the game. Armed with an electric razor and two trash bags, a viral moment was born as the shaving began amid team members, wearing swimsuits with “OUPRIDE!” spelled out in black body paint.

Jonas Cantrell, a junior who assisted with the razor, believes the head-shaving contributed to four missed free throws. “To take that team down to a [66.7] percent free throw average, it showed us we’re doing something,” Cantrell told The Post in February. Cleveland State had a 90 percent free throw average in its previous game.

“Nobody’s making more of a major impact today than the swimming and diving team,” announcer Tony Paul said.

Oakland went on to win the Horizon League tournament last week and earned an NCAA tournament berth for the first time since it went back-to-back in 2010 and 2011. A 14th seed, the Rochester, Mich., school (23-11 overall, 15-5 in the Horizon) will play third-seeded Kentucky (23-9, 13-5 SEC) in a first-round game Thursday night in Pittsburgh.

The freewheeling fun pretty much ends as postseason competition intensifies, moving to neutral sites for conference tournament and NCAA games. At those venues, security is tighter, seating is at a premium and travel can be prohibitive.

Disruptions at Oakland and elsewhere are rooted in, among other examples, those created by Arizona State’s 942 student section and its “Curtain of Distraction” in the fall of 2013. When an empty box large enough to hold a dishwasher or small refrigerator turned up in a storage tunnel at the arena, “we thought, ‘wouldn’t it be funny if we put a student in the box and placed it in front of the student section?’” Bill Kennedy, Arizona State’s associate athletic director and staff adviser of the Student-Athlete Advisory Committee, said. “Then, when the first free throw happens, he pops out of the box just to startle the free throw shooter.”

There was a hitch. “The first free throw didn’t happen for, like, 12 minutes of game time — which is probably roughly the equivalent of probably a good half-hour of regular time. The poor student is in there sweating and then finally his big time comes. They go to the free throw line and he pops out.

“I don’t remember if the free throw was made or missed and we didn’t think much of it after that. But that summer, the 942 [students’] crew was looking at opposing teams who were shooting a pretty high free throw percentage against us in the second half, which is when they shoot toward the student section. They’re up at like 74, 75 percent and the students were trying to think of ways to lower that to give our team a better chance of winning and to create a better home court advantage.”

But the box idea was not “sustainable,” as Kennedy put it. Enter the “Curtain of Distraction,” which created a stir thanks to Phelps. Then assisting with the team while training for the Olympics, Phelps caused a loud reaction “and the person shooting the free throws missed both of them. Then everything just kind of just blew up and it went viral.”

At Duke, Cameron Crazies present more traditional distraction techniques, like arm-flailing and swaying. Nathan Luzum, a medical student, used what he referred to as “back of the envelope” analysis of a limited sample size, gleaned by observation from his attendance at games, to determine which type of distraction may be most effective in getting a shooter to miss.

“From what I’ve seen on TV and in person, the most effective way to induce a miss is some sort of horizontal movement, like to the left or to the right [by fans]. So any time our hands rapidly go up one direction, or there’s some sort of movement of the hands, but back and forth, like at the player’s eye level, that seems to be most effective from what I found,” Luzum said. “There is some amount of human error in [that analysis], but from what I found, this year seems to have continued on the same trend that I saw in the past.”

But creativity is part of the fun. At ASU, other distractions for every men’s and women’s game (as well as a smaller version for road games) have included abduction by an extraterrestrial, actor Charlie Day, Grant Hill, Indy 500 winners, former Arizona Diamondback Archie Bradley, Elvis impersonators, MMA champion Ryan Bader, rapper YG and adoptable puppies.

“This year we had a Jimmy Buffett night and all the skits were themed to his songs,” Kennedy said. “We had a person go out there with a parrot — not a real parrot — and some dancing cheeseburgers and a skit with sharks in it.”

As pure entertainment, the distractions succeed, but it’s difficult to quantify whether the home-court swagger translates into points. Kennedy cited a 2015 Harvard study that, he said, estimated the advantage to be 1.41 points per game.

“Over the course of its 10-ish year history [minus the covid-19 seasons when there were no crowds], teams essentially are making about 67 percent of the free throws, which is down from the 74, 75 before that,” Kennedy said. “So it’s been lower with the curtain there and higher when it isn’t so we like to figure it’s done its job. And even if it doesn’t, it’s a great fan experience and gets students more involved in the game.”