Within the trial, permanent replacements will be permitted for players who suffer head injuries, even if a team has already used up its allotted number of substitutions.
According to IFAB, who are responsible for determining the laws of the game, the change will prevent players suffering further, potentially serious, concussions and sends a strong message about prioritising player welfare.
IFAB also note that the introduction of concussion substitutes will also reduce pressure on medical personnel.
Within the trial, to avoid potential abuse of the rule, the opposition team will also be able to make a change at the same time.