Although not mentioned in the Materia's description, the Preemptive Materia, when mastered and placed on the last character on the party, gives an "Ambush Alert Bonus", which halves the chance of back and pincer attacks.
Each area in the game has a set number of battle formations that are possible to occur during a random encounter. When a random encounter is about to start the game performs a check to see if any of the "special attack formations" apply. If the area has any special formation battles (Back Attack, Side Attack and Pincer Attack) the chance of getting them is calculated first. These battles have the player at a disadvantage, but a mastered Preemptive Materia helps protect against these battles by halving the chances of the disadvantageous ones.
The game uses two types of check in order to detect whether of not a party member has a mastered Preemptive Materia equipped: a single character check and a global check. The single character check is made whenever the player changes Materia or Equipment. If the character does not have a mastered Preemptive Materia the game will remove the Ambush Alert Bonus, which halves the chance of Back and Pincer Attacks, and it doesn't matter if another character does actually have a mastered Preemptive equipped.
The global check is used when entering the menu or exiting the PHS screen by running the single character check on each character in the party in turn, starting from the top. What this means is that only the bottommost character in the party counts for whether they have a mastered Preemptive Materia equipped.
Alert is available from the Guardian Force Cerberus for 200 AP. It doesn't protect against fixed back attacks, such as the one in Deep Sea Research Center against a Ruby Dragon whilst answering Bahamut's questions.
The Alert ability costs four magic stones to equip. It is learned from the Barbut, Ninja Gear, and Germinas Boots. Alert can be learned by Zidane, Steiner and Amarant. The AP to learn Alert depends on the character who is learning it.
Alert is a level 3 Thief ability that costs 3 SP to use and requires 260 JP to learn. It finds enemies, traps, and items on the floor.