Each new heart symbol appearing during the bonus game locks in place and resets your Respins to 3.
The bonus game board expands as you collect heart symbols:
8 hearts: Unlocks 5×5 layout
12 hearts: Unlocks 6×5 layout
16 hearts: Unlocks 7×5 layout
20 hearts: Unlocks full 8×5 layout
The bonus game ends early if you fill the entire board with 40 heart symbols.