There are a few spells and effects that will protect the recipient from incoming spells or attacks. Each can protect against one spell or effect only. They are cast upon the recipient in advance and stay with the recipient until “activated” by the effect they are meant to prevent.
Once a Spell Defense is active, then it will block the next applicable incoming effect, regardless of whether or not you would have been affected by it. It will then be used up. For example, Terin is hit by a Control Undead spell. Not being an undead, it would have no effect upon him. However, it will still use up his Spell Shield.
If a Spell Defense activates while the victim is sleeping normally (not from a Sleep effect), the victim will wake and be aware that the Spell Defense is gone. Spell Defenses will still trigger while a character is unconscious or bleeding out.
All Spell Defenses must be called correctly (for example “Reflect Spell” or “Weapon Shield”) to be effective; these defenses must be called within two seconds of triggering or they will be lost with no effect.
If you forget you have a valid spell defense and then later realize that you did, it is still used up. This is to prevent cheaters from conveniently “forgetting” to use their Spell Defenses except when they really need them.