Any tool is a hammer if you really need one.
The Crescent Hammer is a useful tool added by Thermal Expansion. It is essentially a wrench; it can be used to configure, rotate and/or dismantle various kinds of blocks in the world.
The Crescent Hammer can be used on blocks in the world to configure or rotate them, if they support it. This includes blocks from vanilla Minecraft like Chests, Wood logs and Stairs, Machines and Devices from Thermal Expansion and blocks in other mods that support it.
When the player is sneaking while using the tool on a block, the Crescent Hammer will try to 'dismantle' that block. When a block is dismantled, it is dropped as an item instantly, usually without losing things like stored settings, items or energy.
Most blocks in Thermal Expansion and Thermal Dynamics support dismantling. Other mods may also add support for this on their blocks.