The mini-slot event is a special slot where you win a lot of rewards.

There are three types of spins:

The better the spin, the bigger reward. Every day you receive a free spin, you may win them in the Daily Adventure or purchase them.

While spinning, you may receive additional dice to use on the map on the screen's top to move the character. When you pass the map to the final chest, you will get a bonus reward! This is a fascinating event and one of the best that we have. 
Enjoy it!