Speaking at the England World Cup player welfare briefing, Lindsay Starling, the science and medical manager for World Rugby, said: “It now means that everyone will be able to tell when a player has sustained a significant head injury during the match because their mouthguard will be flashing red. It will create awareness in the stands and among fans on TV.

“It also means there’s absolutely no delay in a player being pulled off the field after sustaining a big head impact because everyone on the field will see they have had a big knock.” This has happened during this World Cup, with six alert events over one weekend of the group stages.

The sensors in the mouthguard can decipher how far someone’s head has moved up or down — this is known as a linear movement — and rotated. The light flashes during occurrences over 75 gravities (the measurement for linear movement) and 4,500 radians (the measurement for angular acceleration, telling us how fast the head has rotated) for men, and 65 gravities and 4,500 radians for women.

https://www.nytimes.com/athletic/6569708/2025/09/18/led-mouthguards-womens-rugby-world-cup/