Crisis Level
Limit Breaks in Final Fantasy VIII are triggered by the event of a character entering "Crisis" status. Several conditions can lead a character to get into Crisis: possessing HP below 32% of their Max HP, being subject to negative statuses (such as Poison, Doom, etc.), being under the Aura status, and/or having one or two allies KO'd. Depending on how serious these conditions are, there are a few Crisis Levels, from 1 (lowest) to 4 (highest). The higher the Crisis Level, the stronger/more effective the Limit Break will be (to see how the Crisis Level affects Selphie's Limit Break in particular, see below, under "Mechanics"). An easy way to get a character into Crisis Level 4 is to lower their HP under 10% of Max HP and cast Aura on them.

Selphie's Limit Break, Slot, is, as hinted by the name, (sort of) like a slot machine that allows the player to indefinitely scroll through random magic spells through the option Do Over. Each spell is associated with a certain number of castings, ranging from one to three; the higher Selphie's Crisis Level, the higher this number will be (and the more rare spells will come up). When you are satisfied with a certain spell, you can choose to Cast it; it should be noted that this does not use up your magic stock: knowing this, it is not hard to conclude that Selphie's Slot allows her to cast magic that she does not even possess in her inventory.

Unique Spells
The following are spells that can be accessed exclusively through the use of Slot.

Full-cure Fully restores HP and cures abnormal statuses to all.
Wall Casts Shell and Protect on all party members.
Rapture Removes enemies from random battles.1
The End Instantly defeats all enemies.2

1Tonberry, Cactuar, Grat, Vysage, Lefty, Righty and E-Soldier are immune to this spell.
2Doesn't work on undead enemies; however, will be effective against all others, bosses included. In multi-phase battles, such as the final one against Ultimecia, it will shift to the next phase instead.

Originally, two other unique spells, called Percent and Catastrophe, were planned to be available through Selphie's Limit Break; for some unknown reason, they were never programmed in, but can still be accessed with a GameShark. If you go to Selphie's fourth status screen through the main menu (the one that lists her special abilities), you can actually see two extra spaces under Full-cure, Wall, Rapture and The End that will always be empty; one can assume they were destined for Percent and Catastrophe. Here are some details on these two missing spells:

Percent Reduces all enemies' HP by a certain percentage.
Catastrophe Ultimate offensive spell.3

3Would have been stronger than even Ultimecia's Apocalypse.

In the Final Fantasy series, the Slot/Slots Limit Break/special ability is usually associated with the Gambler job class. A remarkable example is Setzer Gabbiani from Final Fantasy VI, who is OMFG, LIKE, SO F***ING HOT quite possibly the only full-fledged gambler in the series (you know, with the tragic love story and the wandering spirit and the airship and all); in his case, by selecting Slots you get a slot machine in every sense of the word, for good and for bad: accurately depending on luck, it is even possible to trigger a spell against the party. In a similar situation is Cait Sith from Final Fantasy VII, who possesses a practically identical Slots as his second and last Limit Break. Other characters worth mentioning are Tifa (also from Final Fantasy VII), whose Limit Break depends on a series of reels, and Wakka (Final Fantasy X), whose Overdrive is also called Shots. On a side note, in the mockery of a game that is Final Fantasy X-2, the Gambler class is represented by the Lady Luck dressphere.

Personal Opinion
I'm not a huge fan of Selphie's Limit Break, mainly because I'm not much of a magic user in the first place, but also due to the fact that, in later stages of the game, Full-cure and Wall, depending on how developed your characters are, can become a lot less useful (Auto-Shell, Auto-Protect and Megalixirs, anyone?). As for Rapture and The End, they are both a bit cheap (although the animation for the latter is the best), but some may find them a nice way to end battles, I suppose. It is indeed interesting to note that The End is the only "weakness" that is common to all enemies in the game (except for the undead ones, but then again, if you're that hell-bent on not relying on your characters' skills, Full-cure will take care of those), bosses included; Odin/Gilgamesh's Zantetsuken, on the other hand, doesn't always work. The End actually came up for me during the fight against Omega Weapon, but I decided to skip it because, still in the same line of thought, it's not the most honorable way to finish off the "world's strongest monster", nor the best method to use if you intend to feel good about yourself...

Final Fantasy™ VIII, Selphie and all characters and locations mentioned are ©1999 Squaresoft, now Square-Enix.
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