What are concussion substitutes, and how do they work?

Additional permanent concussion substitutions are an option in the IFAB Laws (Law 3.2) that a competition can choose to adopt; they apply only where the competition's rules allow them. Where they are used:

• each team may make one concussion substitution per match, even if it has used all its normal substitutes
• the concussed player takes no further part, including in a penalty shoot-out
• the other team may then use one additional substitute for any reason
• it does not use up a normal substitution opportunity, unless made at the same time as a normal change
• the team's medical staff and coaches decide; the referee does not judge whether an injury qualifies

Separately, Law 5 treats a head injury such as concussion as a severe injury that can be assessed on the field without the player having to sit out the new one-minute period.