Nails are held in the groove securely by an integral rare-earth magnet. The great advantage of this style is that it lets you hold material with one hand as you use the other hand to start and drive the nail.
A second advantage is that it has shorter than normal claws (of a cat's paw pattern) that substantially increase leverage (while reducing muscle strain) in nail pulling.
With a 20 oz head and a 13" fiberglass handle with a rubber grip, it makes an excellent framing hammer.
Made in Japan.