the handle from being released: the safety lever is spring-loaded, and once the safety pin is removed, the lever will release and ignite the detonator, then fall off. Thus, to use a grenade, the lever is grasped (to prevent release), then the pin is removed, and then the grenade is thrown, which releases the lever and ignites the detonator, triggering an explosion. Some grenade types also have a safety clip to prevent the handle from coming off in transit. To use a grenade, the soldier grips it with the throwing hand, ensuring that the thumb holds the safety lever in place; if there is a safety clip, it is removed prior to use. Left-handed soldiers invert the grenade, so the thumb is still the digit that holds the safety lever. The soldier then grabs the safety pin's pull ring with the index or middle finger of the other hand and removes it. They then throw the grenade towards the target. Soldiers are trained to throw grenades in standing, prone-to-standing, kneeling, prone-to-kneeling, and alternative prone positions and in under- or side-arm throws. If the grenade is thrown from a standing position the thrower must then immediately seek cover or lie prone if no cover is nearby. Once the soldier throws the grenade, the safety lever releases, the striker throws the safety lever away from the grenade body as it rotates to detonate the primer. The primer explodes and ignites the fuze (sometimes called the delay element). The fuze burns down to the detonator, which explodes the main charge. When using an antipersonnel grenade, the objective is to have the grenade explode so that the target is within its effective radius. The M67 frag grenade has an advertised effective kill zone radius of , while the casualty-inducing radius is approximately . Within this range, people are generally injured badly enough to effectively render them harmless. These ranges only indicate the area where a target is virtually certain to be incapacitated; individual fragments can still cause injuries as far as away. An alternative technique is to release the lever before throwing the grenade, which allows the fuze to burn partially and decrease the time to detonation after throwing; this is referred to as cooking. A shorter delay is useful to reduce the ability of the enemy to take cover, throw or kick the grenade away and can also be used to allow a fragmentation grenade to explode into the air over defensive positions. This technique is inherently dangerous, due to shorter delay (meaning a closer explosion), greater complexity (must make sure to throw after waiting), and increased variability (fuzes vary from grenade to grenade), and thus is discouraged in the U.S. Marine Corps, and banned in training. Nonetheless, cooking a grenade and throwing one back is frequently seen in Hollywood films and video games. Tactics Tactics vary by the type of engagement. Urban warfare, particularly the attack of built-up (fortified, buildings etc.) areas, involves the heavy use of hand grenades: typically a grenade or two are thrown before each transition (entering a room or navigating a stairway). A World War II battalion fighting in a city frequently used 500 grenades per day. A key concern is that the grenade is picked up and thrown away or back at the thrower. The USMC's preferred technique to prevent this is a hard-throw, skip/bounce technique, where the grenade is thrown hard enough that it bounces or skips around, being hard to pick up and throw back – this is applicable when clearing a room, for instance. In other uses, such as to reach upper floors of