ULTRA STACK™ BISON INSTANT TRIGGER: Offers immediate feature gameplay with the highest payouts. Free game feature types are available according to play option selected. At the start of any feature, the number of bison symbols on the reels is increased for the free games regardless of play option. When playing the game at higher play options, the top symbol is further increased via a wheel spin. Choosing the highest play option, automatically triggers one of the free game features.