The countdown is a mechanic commonly used in racing games that shows a timer on the screen that counts down. The player must complete a task within this time limit to receive a reward or complete the mission. This mechanic can often be confused with the timer mechanic however that mechanic counts upwards whereas the countdown only counts down.
The first example that does this is ‘Skate 3’. This game is a cooperative game that encourages players to compete against each other by skateboarding. In one of the game modes the players must gain the most points by completing a stunt. However the players only have 2 minutes to do this and a time shows up at the top of the screen counting down. This forces thew player to act quickly and can put them off and cause them to mess up the stunt that they are performing.
Surprisingly this mechanic is this in the racing game mode of Grand Theft Auto 5. when playing online racing against other players the aim is too finish first. Depending on how many people are in the race, after a certain number have finished a 30 second timer will appear in the bottom right, This shows how long the remaining players have to finish or they will receive a dnf (did not finish). This mechanic is extremely effective in this case as it encourages players to perform at the best of their ability and try to win the race.
I feel like the best suite for this mechanic would be in a racing game. Checkpoints would be spread out across the track and the timer would count down continuously throughout the race. As the players pass through the checkpoint it adds extra time to allow them to finish.