Here is a list of the different effects of the slot bonus:

Small- get a 100-900 point bonus
Big- get a 1,000,000-9,000,000 point bonus
Ball Saver- turns the ball saver on under your flippers
Ball Upgrade- you ball upgrades one level
Bonus Multiplier-the number shown will be added to your bonus multiplier
EvoModeStart- starts the evolution mode
Twin Pikachus- gives you left and right pikachu kickback till you loose your ball
Extra Ball(rare)- gives you one extra ball
GoToBonus- proceed to a bonus stage