The Hammer destroys one piece or one obstacle layer (e.g., one Rope out of two) without using a move.
When you want to use this booster, just tap the Hammer icon and then the cell you want to use it on. Please note: to remove one layer of Leaves, use the Hammer on the Leaves or a free piece next to them.
This booster is very handy for getting access to hard-to-reach cells or for when you don't have enough moves left to complete the level goals.
You can buy the Hammer booster when you're already playing a level.