The trench war reinforced the long expressed need for grenades exploding on impact, leaving no opportunity for the enemy to grab it and send it back to the launcher like a conventional "time" grenade. In Germany an original and efficient answer was bring as soon as 1915 with the Discushandgranaten (called "turtle grenades" by the Allied soldiers). The offensive model was made of two thin steel plate shells, crimped together.
The disk grenade had a novel arming mechanism. The grenade contained a cross of four tubes with metal rods that blocked the spring-loaded firing pin. If the grenade was thrown with a spin, the restraining rods moved outward, releasing the firing pin which ignited the fuse. (This is a highly simplified explanation.)
It was made in two versions, an offensive version (.9 lb) for use against a specific target, and a defensive version (.8 lb) with a wide blast spread to defeat attacking waves of infantry.
During 1915, however, the "potato masher" grenade was perfected, which carried a larger charge and did not require as much of a wind-up to throw.
Sources: Quora.com and Passioncompassion1418.com