Proper Use:
Nail hammers are intended for driving and pulling common, unhardened nails, and for ripping apart wooden structures. They are most effective when used to strike the nail head with the center of the striking face. When holding a nail claw hammer place your hand about one inch from the end of the handle. This will provide maximum control, leverage and impact by the hammer head. Place your thumb on top of the hammer handle while gripping the handle for additional accuracy when striking the nail. Use the claw portion of the hammer to remove nails by sliding the nail through the middle of the claw and use the leverage of the curved claw for nail removal.
Mis-Use/Abuse:
Never strike one hammer with or against another hammer or a hatchet. Never strike nail pullers, steel chisels or other hardened objects with a nail hammer as the face may chip, possibly resulting in eye or other serious injury.
Source: Hand Tools Institute, Hand Tool Use and Selection Chart and AMPCO Safety Tools