The open end is put over the top of the post to be driven and moved up and down. The closed end hammers on the top of the post and the large mass aids in sinking it into the ground.
Manual post pounders tend to be around 20lbs, but weight varies with model types. They can be dangerous to handle if raise above the head. Incidentally catching the bottom of the pounder on the top of the post can result in serious injury to the head if the pounder continues to pivot and fall downward.
Hearing protection is advised due to the loud sound produced. It would be wise to wear a hard hat.
An alternative to the manual post pounder was developed in the late 20th century. This device widely known as the skidril is a tool which is driven hydraulically (by oil) the concept allows for a female opening to accept the post being driven. The impacting of the post occurs within a confined space. Impact energy is imparted on the post by a high speed hammering mechanism which prevents recoil and or dangerous pivoting or popping of the driver.