The Serial Boosters help you when you lose a game (come in last place) by erasing the loss and allowing you to maintain your "no-loss" winning streak. At the end of the game, you will be given a choice to use the Serial Booster (or buy one if you are out) to save your current winning streak.
If you choose not to use the Serial Booster you will lose your current winning streak and any additional XP you would have earned for that game.
There are 5 kinds of Serial boosters:
If you lose your winning series and are out of boosters and choose not to buy one OR you choose not to use one that you have, you will return to the beginning where you must then meet the first condition again.
How can I use Serial Boosters?
If you win a minimum of two games, or do not lose a minimum of three games (meaning, you finish the game in second place), and then you lose your third, or fourth game, you can use a serial booster. The first time you lose you can use the Pirate Booster; the second time, the Officer Booster; then the Minister, followed by the king, and finally the Bishop.
If you used the Pirate Booster and you lost your next game again, you can only use the Officer Booster to save your Bonus XP. If you do not have an Officer, but you have a Minister/King/Bishop, you will not be able to use them.
If you lose your winning series (because you don't have more boosters, or you do not want to use a booster, or buy one) you have to win a minimum of two games, or not lose three games to use the boosters again. If your series is interrupted, your first series booster will be the pirate again.
How do I get Serial Boosters?
All Boosters can be purchased when you lose a streak in exchange for Gold.
The Pirate Booster can be obtained through the Pirate's Den building in the Village. The more the Den is upgraded, the greater the number of Boosters can be forged at increasingly shorter intervals. The Pirate Serial Booster can also be purchased in the Pirate's Den.
Similarly, the Officer Booster can be obtained through the University building in the Village. The more the University is upgraded, the greater the number of Boosters can be forged at increasingly shorter intervals. The Officer Serial Booster can also be purchased in the University building.
Currently, the Minister, Prince & Bishop Boosters do not have a building in the Village. They can only be purchased with Gold.