The Type 97 hand grenade, the most common hand grenade used by the Japanese, had an odd mechanism of operation. The striker (with cap and safety pin attached) was attached to the top of the grenade's cylindrical body. To arm the grenade, the cap and safety pin were removed, and the firing pin was screwed all the way down so that it protruded from the bottom of the striker. The exposed firing pin was made to overcome a safety cap and small spring and contact the fuse (starting it) by hitting the striker sharply against a hard surface such as a tile or rock (seen in Hacksaw Ridge)
Japanese explosives for the most part were regarded as substandard by American troops. Being hit by artillery or mortar shrapnel carried a 23 percent fatality risk in Europe, but only 16 percent in the Pacific. The Type 97 hand grenade was often of bad quality, having an erratic fuse and relatively weak explosive charge.
Sources:
G.I.: The American Soldier in World War II, by Lee B. Kennett
Japanese Infantryman 1937-1945, by Gordon L. Rottman
War Department Technical Manual-E 30-480 Handbook on Japanese Military Forces, dated 15 September 1944 (chapter IX, Weapons, subsection Infantry Weapons)