Before any such situation occurs, when they appear on the screen, the punters will have to repel 20 attacking ninja soldiers by throwing their sword star at them. Each ninja you hit can reward up to 27 free spins available, or multiply by 8x.
If you happen to encounter a special ninja, then another 3 ninjas will be eliminated to reward a guaranteed free spin and/or multiplier. But be careful, because some ninjas will quickly avoid your shuriken and end the bonus game. If this happens, the collected multiplier will be applied to all winnings to play the accumulated free spins.