In the Treasure Hunt event, you get boosters as rewards for beating consecutive levels. If you lose a level, the Treasure Hunt will restart from the beginning. Get on a winning streak to start levels with free boosters.
There are 3 Chests with rewards. Each next Chest contains more rewards. Upon reaching the 3rd Chest, you start a level with the boosters from this Chest until you lose a level.
Treasure Hunt usually lasts one or two days.