With the start of World War 1 in 1914, and its associated Trench Warfare battlefields, came the need for various weapons designed to unseat defenders from their positions. The hand grenade, a development of ancient warfare, found renewed value in the modern war and all sides strived to make for better infantry-level grenade weapons in an effort to gain the upper hand.
The typical grenades of the period were fuse-delayed but it was soon found that an enemy could simply grab and throw the grenade back in the direction of the attacker if the fuse ran too long (or failed altogether). As such, attention was given to development of impact-fuze grenades - those that detonated upon impacting a surface and therefore eliminating the threat of having the grenade thrown back.
The Germans managed an ingenious solution through their "Diskushandgranate Model 1915". As its name suggests, this grenade took on a disk-like design in which two sheets of metal (cast iron) were set over one another and crimped at the edges. The metal encapsulated bags containing the required explosives. Ignition was by way of a star-fish like internal assembly that held inertia blocks and percussion pins. A safety pin ensured that the device would not detonate prematurely.
In operation, the safety pin was manually removed to activate the grenade. The operator then was required to throw the disk-shaped object along its longitudinal axis for maximum range. The resultant spinning action pushed the inertia pins into action and the percussion pin was revealed. Upon landing, one of the inertia pins contacted the center of the star shaped structure and activated the starter which led to detonation of the grenade. Fragmentation was made possible through the disintegration of the metal body.
Overall weight was 420 grams, making it one of the lighter German hand grenades of the war. These saw service from 1915 onward and were known to the allies as the "Turtle Grenade" for their easily recognizable shape. In practice, they served their role well for a time but were prone to defects brought about by humidity and other environmental factors found on the battlefield.
Two major versions of the standard design were ultimately manufactured - the standard defensive-minded fragmentation model and an offensive-minded form intended to stun or disorient enemy combatants at closer ranges, typically thrown prior to entering a contested space such as a trench or bunker. The grenades could also be setup in booby-traps as needed which increased their tactical value some.