The Rake removes pieces or one obstacle layer (e.g., one chain out of two) in a whole column and row without using a move.
When you want to use this booster, tap/click its icon and then tape/click the tile that you want to use it on.
This booster is very handy for removing many obstacles all at once in a column and row or when you don't have enough moves left to complete the level goals.
You can buy the Rake booster when you're already playing a level, get it as a reward in events, or win it in the Lucky Spin!