Laydown delivery is a mode of attack using a freefall nuclear weapon: the bomb's descent to the target is slowed by ribbon parachute so that it actually lands on the ground before detonating. Laydown delivery requires the weapon's case to be reinforced so that it can survive the force of impact and generally involves a time-delay fuze to trigger detonation e.g. 45 seconds after hitting the ground. 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. An attack of this type produces large amounts of radioactive fallout.
It has the additional advantage of allowing the carrier aircraft to fly very low and still escape from ground zero without being damaged or destroyed by effects of the nuclear explosion. That is particularly important for high-yield nuclear weapons such as the B83 and B53 nuclear bombs. Low-altitude delivery also helps hide the aircraft from surface-to-air missiles. It was for that reason that laydown was selected for the Vickers Valiant bomber of the Royal Air Force, as the design became increasingly vulnerable to Soviet weapons, especially the SA-2 missile. The low-level laydown delivery was referred to as "Equipment 2 Foxtrot" in RAF parlance; alternatives included "2 Echo" toss bombing and "2 Hotel," a particular climbing delivery method used by the Avro Vulcan.