Secret Cases is a league event that becomes available after completing all regular levels in the game. You unlock puzzle pieces in a grid, and as soon as you collect those, you can play against other players on the leaderboards for the rest of the week. As soon as you unlock Secret Cases, you can play various levels.
Winning a level earns you one point. Players are ranked on the leaderboards based on the number of accumulated points, and the top players with the most points will receive rewards!