With a secret box, team members can drop a note with things they want to discuss in the retrospective whenever something strikes their minds. This way, they won’t forget anything.
This can also be used if people are initially a little reluctant to speak out when the team is in the brainstorming phase. However, the team needs to remember that if an issue needs immediate attention, they should bring it up in the standup meeting or with the team right away instead of waiting until the retrospective. After a period of time, you shouldn’t even need to use the box because every team member should be comfortable speaking openly during retrospectives.