The main bonus feature in Critter Pop is the free spins bonus round, which is triggered when three or more scatter symbols land during the base game. Before the free spins round starts, a Rewards Reel is revealed, bringing with it the chance to win a range of rewards, including extra free spins, a higher starting win multiplier, random 2x2 or 3x3 symbols, random wild symbols and other multipliers, which will be applied to your free spins.
During the free spins round, other features kick in, such as the reels dropping down to the lowest reel height achieved on the current spin. If all the reels end up being eight symbols high, then two further free spins are awarded. However across the entire bonus round players can see an increasing multiplier that starts from x1 by default (can be x2 or x3 if won on the Reward Reel) and grows by +1 with each win. After all reels reach the maximum height that increment grows to +2 for each win, which is an entirely different situation than just "applying an x2 multiplier