Boom

A Collection Meter is a system where a progress bar or gauge (meter) is present on the game screen, which fills up each time the player collects specific symbols or performs certain actions. The core of this feature is to provide the player with a long-term goal that goes beyond the success/failure of a single spin. When the meter is full, an instant prize or a special bonus is awarded as a reward. This provides players with a growth-type fun, similar to 'leveling up' or 'completing a quest'.

Symbol Upgrade is a powerful feature that progressively converts low-value symbols on the reels into higher-value ones as the game proceeds. The core of this feature is not just a one-time symbol change, but a 'growth' structure where lower-tier symbols are permanently removed from the reels and replaced by higher-tier ones. This drastically increases the average expected payout of the reels towards the end of the round.