The NFL’s new helmet rule received its first test on Thursday during the opening preseason game of the year. The rule has caused confusion across the league, with officials and teams alike unable to clarify what is a penalty.

Four minutes into the opening game of the 2018 preseason between the Baltimore Ravens and Chicago Bears, Ravens linebacker Patrick Onwuasor was called for illegal use of helmet after a tackle, resulting in a 15-yard penalty.

“Onwuasor did not seem out of control while bringing down Bears running back Benny Cunningham,” NFL Insider reports.

The replay demonstrated how he placed his head down ahead of contact, prompting the official to throw the flag.

While the rule was put in place to make the sport safer, the use of helmet rule has caused difficulties for players, coaches, and officials attempting to figure out just how it will be enforced .

The rule, which bans players from lowering their helmet to initiate contact but confusingly allows for “incidental contact,” has been criticised as vague by detractors, with Fox rules analyst and former NFL vice president of officiating Mike Pereira saying the rule would be “impossible to officiate.”

Officials efforts to clarify the rule have reportedly fallen short — after the Philadelphia Eagles hosted referees for a presentation regarding the new rule, they were apparently left with more questions than answers.

“The refs were kind of like, ‘Hey, we didn’t make the rules.’ Because I think guys were kind of frustrated,” running back Wendell Smallwood said. “Most of the defense was like, ‘Man, how are we supposed to tackle?’ They were frustrated.”

Conclusively, teams will have to learn on the fly as the rule is enforced through the preseason, and adjust to the officials’ tendencies as necessary.