Final answer:
A slug wrench, also known as a striking wrench, has a circular opening to lock onto a fastener when struck, providing a way to apply significant torque. Mechanics may use a pipe over the wrench's handle for additional leverage, which increases torque but can also lead to bolt or wrench damage if not careful.
Step-by-step explanation:
The term you are looking for is a slug wrench or sometimes called a striking wrench. It is a special type of wrench with a block end on the handle that is designed to be struck. Slug wrenches have an enclosed, circular opening that fits snugly around the fastener. The wrench locks onto the fastener when the wrench is struck, which helps to loosen or tighten it without slipping.
Mechanics sometimes increase their mechanical advantage when using a wrench by adding a length of pipe over the handle of the wrench. This is often done when trying to remove a very tight bolt. The additional length of pipe effectively increases the leverage, allowing the mechanic to apply more torque to the fastener with the same amount of effort. This is similar to the principle of a lever, where the longer the lever, the more force can be applied to the other end. However, this can be hazardous as it may also create enough torque to break the bolt or damage the wrench.