Final answer:
If either team is offside when the shootout begins, the shot should be retaken after ensuring all players are in legal positions. Assistant referees are responsible for monitoring player positions to prevent offside occurrences.
Step-by-step explanation:
If one or both teams are offside when the whistle blows to start the shootout in a soccer match, the proper action according to the rules of the game is to retake the shot. The officials should ensure that all players are in a legal position before the shootout begins. In most cases, only the kicker and the goalkeeper are allowed in the penalty area during a penalty kick. The assistant referees or linesmen typically monitor the positions of the players to prevent any offside occurrence. If an infringement is observed, they must inform the main referee to address the situation immediately. If either team is offside when the shootout begins, the shot should be retaken after ensuring all players are in legal positions. Assistant referees are responsible for monitoring player positions to prevent offside occurrences.