The diamond is the
Scatter symbol. It can be a standard Scatter or the Super Scatter, each being in charge of the Hold and Win and Super Hold and Win bonus games, respectively.
When playing on Level 3, you need at least six Scatters to trigger the bonus game, while on Level 4 you need only four. From there on, it is a classic Hold and Win game; you play with three re-spins that are re-triggered each time you land sticky cash symbols. In the end, you win the sum of the cash volumes.