Laydown delivery is a mode of deploying a free-fall nuclear weapon in which the bomb's fall is slowed by parachute so that it actually lands on the ground before detonating. Laydown delivery requires that the weapon's case be reinforced so that it can survive the impact, and generally involves a time-delay fuse to trigger detonation. Laydown mode can be used to increase the effect of the weapon's blast on built-up targets such as submarine pens, or to transmit a shock wave through the ground to attack deeply buried targets. It has the additional advantage of allowing the carrier aircraft to fly very low – in order to avoid enemy defences – and still get away without being destroyed in the detonation. This is particularly important where high-yield nuclear weapons such as the B41 nuclear bo