A PAL is a Permissive Action Link. This is a generic term for a security device placed on nuclear weapons to ensure that they cannot be used (fired, armed, detonated, etc.) without the possession of a security token (alphanumeric code, key, etc.) from a central command authority. In the event of a nuclear war, codes (or codes to get the codes) must be transmitted or sent from the central command to the forces in possession of nuclear weapons. The U.S. uses coded messages which must match up with a pre-distributed set of codes in order to access the release codes for the weapons - at least, that's what the movies would have us believe.
A truly effective PAL does not simply lock out the electronics of a nuclear weapon. It will, via some physical means, make the physics package unworkable. For example, locking a barrier into place within or across the proper location for the weapon's initiator or physically disconnecting the wiring leading to the conventional explosive detonators - or multiple options thereof. When, for example, nuclear weapons are involved in a BENT SPEAR incident (being dropped out of an airplane by mistake, as has happened) and the military says that several of the 'safety features' prevented the weapon from detonating, these are some of the measures they are describing. In addition to making it difficult for a user to detonate the weapon on command, they will change the physical characteristics of the device so that even massive shock and/or fire makes it difficult to detonate the conventional primer explosives - and if those do detonate, makes a higher-order nuclear detonation less likely.
I can't tell you precisely how they do this, and if I could, I wouldn't. :-)