Contact preclusion is a fuzing feature found in some nuclear weapons in which backup contact fuzes in a nuclear weapon can be disabled when the weapon is set to air burst fuzing.
Air burst has a larger damage radius against soft targets such as personnel or non-hardened buildings than ground bursts due to the Mach stem effect, however in the event the air burst fuze fails to actuate a contact fuze as backup is often included in the weapon.
Though the damage radius will be reduced in this event, it is possible the target will still be destroyed.
[1] Ground bursts produce significant fallout that has the potential to be hazardous to civilians and friendly personnel.
This disables the weapon's backup contact fuze, so if the air burst fuze fails, detonation is precluded, hence the name contact preclusion.