When Miami Dolphins quarterback Tua Tagovailoa appeared to suffer a head injury in Sunday’s game against the Buffalo Bills, it raised questions about the NFL’s concussion protocol. Tagovailoa hit his head on the turf after being pushed by Bills linebacker Matt Milano. Grabbing his helmet with both hands and slowly gathering himself off the turf, Tagovailoa then stumbled and had to be held up by teammates.

He briefly exited the game with what announcers speculated could be a concussion. But he returned after passing the NFL’s concussion protocol and led the Dolphins to a Week 3 win.

Shortly after the game, the NFL Players Association announced it was launching an investigation into Miami’s decision to allow Tagovailoa to return. How does the league’s concussion protocol work and who has the final say about when a player returns to the field?